In a battle between the titleholders and last season's Supporters' Shield winners LAFC, the Sounders came out on top thanks to Morris on Sunday.
The Sounders broke the deadlock in the 11th minute via Raul Ruidiaz's stunning long-range effort at CenturyLink Field in Seattle.
Morris made it 3-0 after scoring twice within a 61-second span to shock LAFC – the Sounders forward outpacing two defenders and dodging the onrushing Kenneth Vermeer for his first of the night.
The American then cushioned home a volley from a Joao Paulo cross in the 49th minute.
Diego Rossi pulled one goal back for LAFC on the hour mark, but it was not enough away from home.
Seattle are second in the Western Conference and two points behind leaders Sporting Kansas City, while LAFC are seventh.
Elsewhere, Nashville edged fellow expansion franchise Inter Miami 1-0 thanks to Anibal Godoy's superb strike.
Godoy curled a shot into the top corner of the net from the edge of the area in the 53rd minute to sink Inter.
Nashville are 10th in the Eastern Conference with two wins from eight games, while David Beckham's Inter are bottom with three points.
Former Manchester United attacker Nani scored and provided an assist in Orlando's 4-1 thrashing of the Chicago Fire on Saturday.
After Chris Mueller opened the scoring, he crossed for Nani to head in a second in the 24th minute.
Given a chance to get back into the game, Chicago squandered it when Alvaro Medran had a penalty well saved by Pedro Gallese.
Chicago took their next chance from the spot through Robert Beric early in the second half, but Orlando sealed their win late.
Junior Urso scrambled in from inside the area after Nani's set-piece went uncleared, and the Portuguese attacker's pass later found Benji Michel to finish off in additional time.
Orlando are second in the Eastern Conference, five points behind the high-flying Columbus Crew.
Second-half goals from Pedro Santos and Gyasi Zardes saw Columbus record a 2-0 win over Nashville.
Diego Valeri netted a brace as the Portland Timbers crushed the San Jose Earthquakes 6-1 at Avaya Stadium.
Fresh off confirming the signing of Gonzalo Higuain, Inter Miami recorded a second win in three matches by beating Atlanta United 2-1.
Elsewhere, a 10-man LA Galaxy fell to the Colorado Rapids 2-0 and Dallas also finished with 10 but recorded a 3-2 win over Sporting Kansas City.
Haris Medunjanin scored directly from a corner in the 85th minute as Cincinnati beat the New York Red Bulls 1-0 and the Vancouver Whitecaps beat a 10-man Real Salt Lake 2-1.
New York City drew 0-0 with the New England Revolution, DC United and Toronto played out a 2-2 draw, while the Houston Dynamo and Minnesota United did likewise.
Former Manchester United winger Nani scored his ninth goal of the MLS season as Orlando City missed the chance to close the gap at the top after a 1-1 draw at Cincinnati on Saturday.
Cincinnati, who were winless in their past seven games coming in, opened the scoring in the 42nd minute from Brazilian attacker Brenner.
Nani levelled moments after being brought into the game as a 55th-minute substitute, driving forward and firing in a low left-foot pinpoint shot.
The result means back-to-back draws for Orlando who are second in the Eastern Conference, seven points behind leaders New England.
Third-placed New York City blew an early two-goal lead to draw 2-2 away to Toronto.
Santiago Rodriguez and Ismael Tajouri had put City up 2-0 inside 21 minutes but Toronto netted twice after the break.
Richie Laryea pulled one back in the 49th minute before reigning MVP Alejandro Pozuela equalised in the 55th minute with a tap in.
Portland Timbers beat Real Salt Lake 3-2 in a see-sawing contest with Felipe Mora netting the eventual winner.
Columbus Crew suffered their third straight defeat as Ezequiel Barco netted a double for Atlanta City in a 3-2 win.
Dallas continued their good run of recent form, claiming their third win in four games, with a 2-0 triumph over Austin.
Minnesota United won 2-0 over Houston Dynamo, while Colorado Rapids drew 0-0 with Western Conference leaders Sporting KC.
Bruce Arena's side, who clinched their first-ever Supporters' Shield on Saturday, won thanks to Tajon Buchanan's 74th-minute strike.
The result took Revolution's season tally to 73 points from 33 games, eclipsing the previous record mark held by LAFC with 72 points in 2019.
New England have one more regular season match to extend their record, playing Inter Miami at home on November 7.
Canada international Buchanan came up with the winner, showing fast feet to make space for a low shot that beat Rapids goalkeeper William Yarbrough, who had sensationally kept out Henry Kessler from close range minutes earlier.
New York Red Bulls slipped out of the Eastern Conference play-offs spots after a 1-0 defeat to DC United who moved above them into seventh place.
United snapped a four-game winless skid, while also ending the Red Bulls' four-game running run and eight-game unbeaten streak in the process. Kevin Paredes side-footed home the ninth-minute winner for United.
Sporting KC moved into top spot in the Western Conference and dented LA Galaxy's play-offs aspirations with a 2-0 win.
Johnny Russell extended his scoring run to eight straight games with both goals for KC who clinched a top-four seeding as well.
Randall Leal scored two goals and grabbed an assist as Nashville came from two goals down twice to win 6-3 at Cincinnati.
Portland Timbers moved in the west's top four with a 2-0 home victory over San Jose Earthquakes, while Vancouver Whitecaps climbed up to fifth with a 2-1 triumph over Minnesota United.
New York City boosted their Eastern Conference top-four seed hopes with a 1-0 home win over Chicago Fire, while Josef Martinez's winner helped Atlanta United past Inter Miami 2-1 after Gonzalo Higuain had opened the scoring from the spot.
Columbus Crew upset Orlando City 3-2, Toronto held Philadelphia Union 2-2 as Jozy Altidore netted while Real Salt Lake scored twice in the final 10 minutes to win 2-1 at Dallas.
The Revolution improved their record, with back-to-back wins, to the best in the MLS with 30 points from 15 games, sitting atop the Eastern Conference.
Inter are bottom of the east, with only eight points from 14 games, having lost their past six.
Carles Gil provided the assist, his 11th of the season, for Traustason's 15th-minute header, before Teal Bunbury fired in a 27th minute goal from Inter goalkeeper Nick Marsman's parry.
Icelandic midfielder Traustason grabbed his second in the 36th minute, whipping in a shot after a goalmouth scramble from a corner.
Gil was crafty on the right side of the box before setting up another goal with Adam Buksa lashing home a fine volley on the stroke of half-time.
Buksa grabbed a double in the 83rd minute as Gustavo Bou put him into space with the Polish striker finishing with class.
Without Javier Hernandez, LA Galaxy came from two goals down to claim a 2-2 draw away to Real Salt Lake.
Albert Rusnak and Maikel Chang had scored inside the opening 25 minutes to give RSL a 2-0 lead.
Victor Vazquez pulled one back with a top-corner strike in the 33rd minute, before RSL keeper David Ochoa had spectacularly saved a second-half Rayan Raveloson header.
But Madagascar talent Raveloson headed in a 77th-minute equaliser to earn a share of the spoils.
DC United scored twice in the final 10 minutes to rescue a 2-2 draw at struggling Chicago Fire.
Gaston Gimenez's 32nd-minute strike, on his return from the Copa America, and Andy Najar's 61st-minute own goal had Chicago on track but United hit back late.
In the 82nd minute, Yordy Reyna laid off for Kevin Paredes who fired in off the post, before Ola Kamara converted an 87th-minute penalty.
Hungarian forward Daniel Salloi netted a 94th minute equalizer for high-flying Sporting KC to claim a 1-1 home draw with 10-man San Jose Earthquakes who are winless in 10.
Salloi forced home a rebound after Johnny Russell's free-kick hit the woodwork after Nathan had headed the Earthquakes ahead from a corner against the run of play early in the second half.
Felipe Mora glanced on a late winner for Portland Timbers to edge Los Angeles FC 2-1 after Carlos Vela had earlier squared up following Diego Valeri's 100th goal for the club.
Colorado Rapids eased past Dallas 2-0 to extend their unbeaten run, Cincinnati drew 1-1 with Atlanta United and Columbus Crew and Nashville played out a goalless draw.
Toronto drew 1-1 with New York Red Bulls, while New York City edged Montreal 1-0.
After failing to score in their opening two games – a 1-0 loss to LA Galaxy and a goalless draw with Vancouver Whitecaps – City hit the lead after only seven minutes, with Alexander Callens rifling home after Montreal failed to clear their lines from a corner.
Santiago Rodriguez spectacularly chipped Sebastian Breza to double the margin on 20, but only after Montreal gifted City possession in an attempt to play out from their own half.
Zachary Brault-Guillard pulled one back for CFM, but following further City goals from Telles Magno and Thiago Andrade, the scoreline arguably belied the performance.
Douglas Costa opened his account in the MLS later on Saturday, scoring the equaliser in LA Galaxy's 3-2 loss away to Seattle Sounders.
Javier Hernandez gave the Galaxy a shock lead in the sixth minute, after fine link-up play between Raheem Edwards and Kevin Cabral, but the Sounders fought back to lead before the interval – with goals from Jordan Morris and Fredy Montero.
Costa drew the visitors level from the dead ball after 48 minutes, wrong-footing Stefan Frei, but Xavier Arreaga headed home in the 72nd to give the Sounders their first win of the season.
Los Angeles FC continued their unbeaten start to the season, meanwhile, grinding out a 2-0 win against a 10-man Inter Miami at DRV PNK Stadium.
A deflected Carlos Vela pass put Kwadwo Opoku through on the stroke of the interval, and the 20-year-old Ghanaian finished with composure. That didn't spell the end of events in the first half, though, as Brek Shea was given a straight red card in injury time.
LAFC had six shots for the match compared Inter's seven, despite 53 per cent possession and an extra man. However, an Ismael Tajouri free-kick crept inside Inters back post in the 82nd minute to secure the win.
Steve Cherundolo's side are equal top in the Western Conference with Real Salt Lake, who defeated New England Revolution 3-2. Austin FC couldn't keep pace in the final Saturday game, however, going down 1-0 away to Portland Timbers through a Bill Tuiloma goal.
Columbus Crew share the lead in the Eastern Conference, following their 2-1 win over Toronto FC. They are joined by Philadelphia Union, who defeated San Jose Earthquakes 2-0.
A Darwin Quintero brace handed Houston Dynamo a 2-0 win against Vancouver Whitecaps, while Brandon Vazquez scored a double of his own in FC Cincinnati's 2-1 win at Orlando City.
FC Dallas, Chicago Fire and Colorado Rapids respectively secured 2-0 victories over Nashville, DC United and Sporting Kansas City in Saturday's other games.
Mitrita led the way for New York after opening the scoring just 30 seconds into the match at Red Bull Arena on Saturday.
It was the fastest goal in New York's history and quickest in MLS since Allan Cruz's 28-second effort for Cincinnati in 2019, per Opta.
After Cincinnati went close to equalising, Anton Tinnerholm doubled New York's lead from an acute angle in the 25th minute.
Mitrita made it a brace two minutes before half-time and Jesus Medina completed the scoring with two minutes remaining as New York moved up to fifth in the Eastern Conference, five points behind fourth-placed Toronto.
Elsewhere, Maynor Figueroa's 89th-minute equaliser rescued a 1-1 draw for Houston against Nashville.
Daniel Rios' 49th-minute goal had Nashville on track for maximum points until Figueroa scored from a set-piece situation.
Houston are ninth in the Western Conference, while Nashville sit ninth in the east.
Alexandre Pato's 94th-minute free-kick cannoned into the crossbar, with Andres Perea forcing home the rebound, ushering in scenes of celebrations as it would have sealed a post-season berth for Orlando.
That jubilation turned into frustration when the goal was later disallowed after referee Allen Chapman consulted the video, deeming Daryl Dike committed a foul on Alistair Johnston amid the goal-line scramble.
"It's a very sad day for the league," Orlando head coach Oscar Pareja said. "There is no explanation on a play that we saw evident. And it destroyed the joy of our people."
United States international Dike had earlier opened the scoring in the 18th minute, before Hany Mukhtar levelled it up for Nashville eight minutes into the second half.
Orlando – fifth in the Eastern Conference – can still reach the play-offs if they beat Montreal on the final day of the season on Sunday, while Nashville are third.
Philadelphia Union moved above Nashville into second in the east with their routine 2-0 home victory over struggling FC Cincinnati.
The Union are ahead of Nashville on goal difference only, with strikes from Daniel Gazdag and Paxten Aaronson securing the win.
Cincinnati are eight points adrift at the foot of the table and have lost 11 games in a row and 14 of their past 15, while they have only one win in their past 24 games.
Elsewhere, Minnesota United stayed firmly in the play-off hunt thanks to a 2-1 home win over Western Conference leaders Sporting Kansas City.
Dominique Badji's 58th-minute goal was enough for Colorado Rapids to edge Houston Dynamo 1-0, meaning they tied their club record for points in a season with 58.
The Galaxy, who were without Javier Hernandez due to injury and Jonathan dos Santos, Efrain Alvarez and Sebastian Lletget to Gold Cup call-ups, were brushed aside by Dallas who were last in the Western Conference prior to the game.
Pepi, who turned 18 in January, becomes the youngest player in MLS history to net a hat-trick.
The teenager netted twice in the first half before completed his treble with a low shot from the edge of the box in the 50th minute.
Substitute Jader Obrian added another in the 88th minute as Dallas moved up to 10th in the west, while the Galaxy are third but are winless in their past three.
Carlos Vela got on the scoresheet as LAFC came from two goals down to rescue a 2-2 home draw with struggling Vancouver Whitecaps.
Colombian pair Deiber Caicedo and Cristian Dajome had put the Whitecaps two-up inside the first 38 minutes, before Vela pulled one back in the 40th minute.
Diego Palacios provided the pass for fellow Ecuadorian Jose Cifuentes to level in the 75th minute.
Charles Sapong scored twice as Nashville SC moved up into second spot in the Eastern Conference with a 3-0 victory over Cincinnati.
Toronto maintained their good recent run with a 2-1 win at Chicago Fire, while Minnesota United scored twice in the final 20 minutes to win 2-1 over Portland Timbers.
San Jose Earthquakes drew 1-1 with Houston Dynamo and Real Salt Lake won 3-0 over Colorado Rapids.
Union, who are fifth in the Eastern Conference, went behind in the 10th minute when Przemyslaw Frankowski headed in Boris Sekulic's cross.
Chicago were reduced to 10 men when Wyatt Osmberg received a straight red card in the 34th minute, after VAR review, and Philadelphia capitalised from the ensuing free-kick.
German defender Kai Wagner scored the equalizer with a shot which went under the jumping Chicago wall.
Daniel Gazdag spurned a chance for Philadelphia with a header which went wide in the 52nd minute, while Union substitute Ilsinho fluffed a late volley.
Philadelphia have now won only one of their past seven games.
Nashville edged their way up to third in the Eastern Conference with a 1-1 draw at lowly Toronto.
Omar Gonzalez had headed Toronto ahead from a 19th-minute quick free-kick, but Robert Castellanos levelled before half-time from a long throw-in.
Union scored first-half goals from Alejandro Bedoya and Daniel Gazdag, while Sergio Santos had a goal disallowed on the stroke of half-time, as Philadelphia got some revenge after last season's Eastern Conference final defeat to City.
Bedoya put Philadelphia ahead in the 12th minute, but City thought they had an avenue back into the game when referee Ted Unkel awarded them a penalty and sent off Jack Elliott for a 28th-minute last-man push on Santiago Rodriguez.
However, VAR intervened and determined that Rodriguez had illegally used his hand in the lead-up, meaning the penalty and red card were overturned.
The win was Philadephia's first at Yankee Stadium and came despite City bossing the possession count with 74 percent.
Former Liverpool and Bayern Munich attacker Xherdan Shaqiri netted his first MLS goal as Chicago Fire won 3-1 over Sporting KC.
Shaqiri scored from the spot in the 50th minute to make it 2-0 as Chicago extended its unbeaten start to the new season with Kacper Przybylko hitting a double.
Jesus Ferreira scored a first-half hat-trick as Dallas defeated Portland Timbers 4-1 in a sign of intent for the forward ahead of USA's upcoming World Cup qualifiers.
All three of Ferreira's goals came in a 10-minute span, marking his first-ever MLS hat-trick.
Gonzalo Higuain scored, but Inter Miami remain winless this term after a 3-1 loss away to last season's strugglers Cincinnati.
The result leaves Phil Neville's side with an MLS-worst record of one point from four games, having conceded 10 goals and only scored two.
Karol Swiderski netted a double as Charlotte upset New England Revolution 3-1, while Luis Amarilla's 32nd-minute strike lifted Western Conference leaders Minnesota United past San Jose Earthquakes 1-0.
Tyler Pasher's 90th minute goal salvaged a 1-1 home draw for Houston Dynamo against Colorado Rapids, while 10-man Atlanta United scored twice in the final five minutes to rescue a 3-3 draw with Montreal.
Facundo Torres' early goal got Orlando City past LA Galaxy 1-0, Bob Bradley's Toronto knocked off DC United 2-1 and Real Salt Lake edge 10-man Nashville 2-1.
Javier 'Chicharito' Hernandez scored twice for the Galaxy in a 3-3 draw with Minnesota United on Decision Day, but RSL's last-gasp 1-0 win over Sporting Kansas City changed the landscape of the Western Conference.
RSL pipped the Galaxy – without post-season football for the second straight campaign – to the seventh and final play-off spot on victories for the season via a 95th-minute winner, having finished level on 48 points at the end of 34 rounds.
Minnesota finished fifth to clinch a post-season berth in the west, along with sixth-placed Vancouver Whitecaps, who drew 1-1 against Seattle Sounders.
Seattle's stalemate and Sporting KC's late loss allowed Colorado Rapids to secure top seed in the Western Conference following a 5-2 rout of Los Angeles FC, who were eliminated as a result.
In the Eastern Conference, Orlando City beat Montreal 2-0 for a spot in the play-offs, along with Atlanta United (2-1 over FC Cincinnati) and New York Red Bulls – the latter using a 1-1 draw against Nashville to extend their post-season streak to 12 years.
Supporters' Shield winners New England Revolution lost 1-0 to already eliminated Inter Miami, Portland Timbers eased past lowly Austin 3-0, DC United defeated Toronto 3-1, 10-man New York City drew 1-1 with Philadelphia Union and reigning champions Columbus Crew – who will not defend their title – beat Chicago Fire 2-0.
Chris Wondolowski scored in the San Jose Earthquakes' 1-1 draw against Dallas before the league's all-time leading scorer announced his retirement.
Fabio scored the 79th-minute winner, his second goal of the season, after Coronel had saved Mauricio Pereyra's penalty on the stroke of half-time.
Venezuelan midfielder Cristian Casseres fired in an excellent left-foot strike to open the scoring in the sixth minute but Orlando had a chance to level moments before half-time.
Uruguayan midfielder Pereyra shot low to Coronel's left but the Brazilian dived well and palmed away the spot-kick.
Pereyra partly made up for his miss, laying off the pass for Chris Mueller to equalize high into the back of the net in the 59th minute.
But Fabio held off a defender to fire home to secure the Red Bulls' first road win of the season, moving them up to seventh in the Eastern Conference.
DC United had seven different scorers as they trounced Toronto 7-1, keeping them ahead of the Red Bulls in sixth in the East.
Andy Najar, who did not score one of United's goals, set up two and was named Player of the Match.
Eastern Conference leaders New England Revolution blew a two-goal lead to draw 2-2 away to Columbus Crew.
Goals from Tajon Buchanan and Gustavo Bou had New England on the path to victory but Columbus fought back with Andrew Farrell's comical own goal levelling it up.
LAFC grabbed their maiden road win of the campaign, edging Real Salt Lake 1-0 after Jose Cifuentes' 69th-minute strike from Eddie Segura's pass.
Segura was the hero moments later with a diving goal-line clearance to deny Rubio Mendez's on-target lob.
Houston Dynamo and Cincinnati traded goals inside the opening five minutes in a 1-1 draw. Houston have drawn four games in a row.
Argentine forward Ignacio Aliseda scored twice as Chicago Fire claimed their second win of the season, triumphing 3-0 over Atlanta United.
Inter Miami's struggles continued with a 1-0 defeat away to Montreal. Miami have only managed eight points from 11 games, losing their past five.
Benjamin Kikanovic scored late to earn a 2-2 draw for struggling San Jose Earthquakes at Minnesota United. The Earthquakes are winless in eight.
Charles Sapong's second-minute goal was enough for Nashville to secure a 1-0 home win over fellow Eastern Conference contenders Philadelphia Union.
Goals from Brazilian forward Fabio and defender Kyle Duncan either side of half-time settled Friday's contest for the Red Bulls, who recorded their fourth consecutive home victory.
The result inflicted Nashville's first defeat of the season, also seeing them slide down to eighth in the east as the Red Bulls moved up to third – five points behind leaders New England Revolution.
Fabio, who joined the Red Bulls in February, netted his first goal for the club with a fine volley in the 37th minute after Patryk Klimala's crafty flick-on.
Ten minutes into the second half, Duncan added a second, collecting the ball at halfway, running forward and unleashing a low left-foot finish from outside the box.
Real Salt Lake scored twice in stoppage time to end their three-game run of draws courtesy of a 3-1 victory over Vancouver Whitecaps.
Erik Holt and Damir Kreilach struck at the death, helping Real Salt Lake move up to fifth spot in the Western Conference.
The late double blow also condemned the Whitecaps to their fourth successive defeat.
Kreilach opened the scoring in 43rd minute following Albert Rusnak's classy pass, but Vancouver levelled 10 minutes after half-time from an unmarked Brian White at the back post.
The game appeared destined for a draw before Holt headed home from a long Aaron Herrera throw into the box in the 92nd minute, with Kreilach nodding in another to seal the win three minutes later.
Clark – aged 17 years, four months and 18 days – became the youngest player to score in his first two MLS appearances on Wednesday, per Opta.
Hours after joining the Red Bulls, Clark capped his debut with a goal in Saturday's 1-0 win over Atlanta United.
Clark became the youngest player to score his maiden goal for the Red Bulls since Jozy Altidore in 2006.
He continued where he left off against Toronto, netting the equaliser with 13 minutes remaining midweek.
Following Alejandro Pozuelo's 23rd-minute penalty, Clark earned the visiting Red Bulls a point by firing a shot into the top corner of the net from outside the box.
The Red Bulls are seventh in the Eastern Conference, while Toronto – already play-off bound – remain top through 18 games.
Elsewhere, reigning Supporters' Shield holders Los Angeles FC and struggling LA Galaxy were beaten.
Lucas Cavallini's brace lifted Vancouver Whitecaps to a 2-1 victory against fourth-placed LAFC.
Canada international Cavallini has now scored in back-to-back games after finding the net just twice in his first 11 MLS matches for the Whitecaps, who climbed up to eighth in the standings and in the play-off positions.
As for the Western Conference's bottom side – the Galaxy – they were swept aside 4-0 by San Jose Earthquakes in the Cali Clasico.
Former Manchester United and Real Madrid striker Javier Hernandez was substituted early in the second half, with the Galaxy only managing four goals in the 652 minutes 'Chicharito' has been on the pitch this season.
It was a sixth consecutive defeat for the slumping Galaxy as the Earthquakes moved within a point of LAFC in the west.
Wednesday's scheduled Minnesota United-Chicago Fire fixture was postponed due to a suspected coronavirus case among Minnesota's team delegation.
In other results, New England Revolution edged Montreal Impact 3-2, FC Cincinnati were 2-1 winners over Columbus Crew, Orlando City and New York City drew 1-1, Atlanta saved a 1-1 draw at Inter Miami, Sporting Kansas City fell 1-0 to Dallas, DC United and Philadelphia Union shared the points in a four-goal clash, Real Salt Lake trumped Portland Timbers 2-1 and Nashville accounted for Houston Dynamo 3-1.
Adam Buksa's goal after 70 minutes kept New England atop the east with a 1-0 win over FC Cincinnati.
The Revolution are now five points clear of second-placed Orlando City after the Lions' 2-1 defeat to New York Red Bulls.
Ten-man New York City FC stunned Los Angeles FC 2-1 on Ismael Tajouri-Shradi's 90th minute goal after Jesus Medina had equalised 20 minutes earlier.
Trailing 2-0 heading into the final 10 minutes, Nashville saw Hany Mukhtar score in the 80th and 83rd minutes to lift the visitors to a 2-2 draw with Atlanta United.
Mason Toye's 87th-minute strike gave Montreal a 1-0 victory over Chicago Fire, while Columbus Crew held on to beat Toronto FC 2-1 on early goals by Luis Diaz and Gyasi Zardes.
Sporting Kansas City rallied from a first-half deficiit to defeat Houston Dynamo 3-2 as Gianluca Busio, who turned 19 on Friday, started the comeback with an outrageous free kick.
Ola Kamara's brace led the way as D.C. United rolled 3-0 to keep Inter Miami winless at home.
A late own goal from Tanner Beason of the San Jose Earthquakes gave LA Galaxy a 1-0 win.
Niko Hansen's 78th-minute tally salvaged a 1-1 draw for Minnesota United against Real Salt Lake after Damir Kreilach opened the scoring in the 13th minute.
Colorado Rapids got goals from Keegan Rosenberry, Tanner Tessmann and Michael Barrios to ease past last-place FC Dallas 3-0.
Bou struck at the death in the play-in match at Gillette Stadium, where the Revolution earned a date with Philadelphia Union in the opening round of the MLS Cup play-offs.
The Argentine forward's goal came on his eighth shot of the match, the most by a Revolution player in a play-off match in club history, per Opta.
Bou's match-winning effort – officially timed at 94 minutes, 37 seconds – is New England's latest winning goal since Kelyn Rowe at 95:47 in 2017.
The tie was level after Montreal's Romell Quioto cancelled out Carles Gil's 38th-minute opener just past the hour mark – the Impact scoring in 21 consecutive away matches (including play-off games), the longest streak in MLS history.
In the day's other play-in fixture, Nashville eased past David Beckham's Inter Miami 3-0.
In a battle of the 2020 expansion sides, Nashville progressed to face Toronto thanks to goals from Randall Leal, Hany Mukhtar and Dax McCarty.
A two-goal first half, fuelled by Leal and Mukhtar, set the tone as Nashville became the first true expansion team to win a play-off match since Chicago Fire in 1998, according to Opta.
Supporters' Shield leaders the Revolution booked their play-off spot courtesy of a 90th-minute winner from MVP candidate Carles Gil on Wednesday.
New England – 15 points clear atop the Eastern Conference and 10 points ahead in the Supporters' Shield race – entered the midweek clash amid a four-match unbeaten streak against Chicago – the Revolution's longest undefeated run versus the Fire.
They extended that streak in an entertaining showdown with hosts the Fire at Soldier Field in Chicago.
Henry Kessler opened the scoring for the Revs in the 11th minute before Chicago's Carlos Teran restored parity entering the interval.
The Revs and Fire traded goals via Teal Bunbury and Gaston Gimenez before substitute Gil struck at the death.
Nashville, who are second to New England in the east, crushed David Beckham's Inter Miami 5-1.
Inter Miami's club-record six-match unbeaten streak (W5 D1) ended with a 4-0 rout at the hands of New York Red Bulls last time out – it was only the club's second defeat in 12 matches following a six-game losing run.
But Inter Miami crashed to consecutive humiliating defeats in a match that saw Gregore sent off with four minutes of regulation remaining.
Hany Mukhtar's brace fuelled visiting Nashville, with Inter Miami ninth in the Eastern Conference and three points adrift of the play-off positions.
Meanwhile, the derby honours were shared between the Red Bulls and 10-man New York City following a dramatic 1-1 draw.
Patryk Klimala converted a 102nd-minute penalty to salvage a point for the Red Bulls.
Reynoso scored twice inside the opening 20 minutes to set up the triumph which lifts United up to sixth spot after they had suffered back-to-back defeats.
The Argentine fired in a low left-foot shot to open the scoring in the fourth minute before jinking inside and netting from similar range in the 20th minute to double the lead.
United goalkeeper Tyler Miller saved well to deny Dejan Joveljic prior to half-time, before Ethan Finlay got in behind the Galaxy defence from Franco Frangapane's pass to add a third in the 66th minute.
The result sees Minnesota move up to sixth in the Western Conference with 34 points, while Galaxy stay fourth but are winless from their past six games.
Runaway league leaders New England were held to a 1-1 home draw with Columbus Crew.
Polish attacker Adam Buksa netted a 62nd minute equalizer after US international Gyasi Zardes had put Crew ahead in the 58th minute.
Zardes had missed a gilt-edged header in the 22nd minute but he took his opportunity early in the second half from Harrison Afful's cross.
Buksa levelled, swooping on a poor Jonathan Mensah pass in defence, handing the Pole a sniff on goal which he converted with a left-foot shot.
An injured Eloy Room made two crucial late saves to deny Buksa a winner as the Crew held on.
Struggling Austin slumped to a fifth straight defeat after blowing a 3-1 second-half lead to lose 4-3 to San Jose Earthquakes.
The Earthquakes scored three goals in 14 second-half minutes to complete the comeback headlined by Javier Lopez's double.
New York City firmed up their grip on a playoffs spot after a 2-1 win away to Cincinnati who were reduced to nine men after red cards to Alvaro Barreal and Isaac Atanga.
Damir Kreilach's early second-half goal was enough to get Real Salt Lake past Seattle Sounders 1-0, while Houston Dynamo won 3-2 over Dallas in the Texas derby.
Lowly Toronto ended their six-game losing run with a shock 2-1 home win over Nashville, while Atlanta United secured their third straight triumph with a 3-2 victory over DC United.
Australian full-back Brad Smith provided an assist for Ruidiaz's 20th-minute opener, before scoring a goal himself three minutes later, forcing home a rebound.
Ruidiaz settled the contest in second-half stoppage time when he fired home on his left foot from substitute Jimmy Medranda's low cross.
The Sounders dulled the threat of Galaxy forward Javier 'Chicharito' Hernandez – who had scored five goals in his opening two games – in a strong display, where they marginally lost possession but had nine to two shots on target.
The win moves Seattle to seven points from three games to start the new campaign, while the Galaxy suffered their first blemish of the campaign after scoring six goals in their opening two games.
Inter Miami drew a blank in the absence of the Higuain brothers with a 0-0 draw away to Nashville.
Former Juventus and Real Madrid forward Gonzalo Higuain and brother Federico were unavailable for the match after flying back to their native Argentina following the death of their mother to cancer.
The Higuain brothers' team-mates offered a wonderful sign of support ahead of the game.
Inter Miami – co-owned by former Manchester United, Madrid and England star David Beckham – have four points from three games, while Nashville are still winless but have drawn all three matches this season.
Diego Rubio's incredible free-kick helped Colorado Rapids to a 1-0 victory away to Vancouver Whitecaps.
Rubio struck in the 26th minute with a curling right-foot shot that found the top corner, going in off the post and crossbar.
The spectacular strike earned the Rapids their first three points of the season, while condemning the Whitecaps to their first defeat.
