The Kings, after winning the toss and batting first, posted 177-7 from their 20 overs.
Avishka Fernando was the main contributor for the three-time champions with 59 from 48 balls while Pathum Nissanka hit 51 off 33 balls.
Fernando’s innings included seven fours and a six while Nissanka hit six fours and two sixes.
Captain Charith Asalanka also contributed a 15-ball 33 against 3-34 off four overs from Zahoor Khan and 2-23 from four overs from Dwayne Pretorius.
The Marvels then used all of the 120 balls available to them to successfully chase down their target.
Alex Hales led the way with 65 off 47 balls including seven fours and two sixes while Captain Niroshan Dickwella made 47 off 27 balls including eight fours and a six as Galle reached 179-5 off 20 overs.
Janith Liyanage also made a crucial 13-ball 25 for Galle. It eventually came down to them needing three runs off the last ball, a ball that Sahan Arachchige hit for four to seal the win.
Fabian Allen took 2-33 from his four overs for the Kings.
Full Scores:
Jaffna Kings 177-7 off 20 overs (Avishka Fernando 59, Pathum Nissanka 51, Charith Asalanka 33, Zahoor Khan 3-34, Dwayne Pretorius 2-23)
Galle Marvels 179-5 off 20 overs (Alex Hales 65, Niroshan Dickwella 47, Janith Liyanage 25, Fabian Allen 2-33, Avishka Fernando 2-40)
Sri Lanka won the game by 161 runs but more importantly, the hosts now hold an unassailable 2-0 lead in their three-match ODI series.
The hosts benefitted from centuries of 127 and 119 from opener Avishka Fernando and Kusal Mendis respectively, the pair helping them to a mammoth 345-8 from their 50 overs. The West Indies did not get close, as only Shai Hope, 51, and Roston Chase, 31, offered any resistance.
The visitors would end on 184 all out against the bowling of Wanindu Hasaranga, 3-30, Lakshan Sandakan, 3-57, and Nuwan Pradeep, 2-37. There was also a wicket for Angelo Matthews, 1-20.
But Pollard did point out that things could have been different had he not dropped Mendis early in the innings and maybe, the series would now be different.
"It was the start we were looking forward to, getting two wickets early. But I put the catch of Kusal Mendis down, otherwise they would have been three down,” said Pollard.
In addition, Pollard believes his side did not have faith in the plans they made.
“We didn't stick to our plans long enough. I don't think we were up to the international standard today. Well bowled to them and well played to them,” he said.
Pollard will now look toward the final game of the series to see if the West Indies can win one.
“We weren't there in all three departments today. We can pinpoint a lot of things but I don't want to go into a lot of details. We'll do that as a team. I cop this one on the chin and we move on to Kandy."
After Allen took one wicket for 26 runs to assist in restricting the Strikers to 188-8, Rossouw exploded in the Kings turn at bat, smashing 12 boundaries and six maximums in a 50-ball knock that secured victory at 190-3 with nine balls to spare, at Dambulla.
Scores: Colombo Strikers 188-8 (20 overs); Jaffna Kings 190-3 (18.3 overs)
After electing to bowl first, the Kings made early inroads, as Asitha Fernando removed Rahmanullah Gurbaz, who made a quick fire 11-ball 27, with the score at 34-1.
Allen took 1-26 from four overs.
Glenn Phillips, with a 32-ball 58, and Angelo Perera, who was left unbeaten on 34, added 86 for the second wicket, before the latter retired hurt. Phillips’ struck six fours and three sixes in his top score, before he fell in the 15th over.
Following his dismissal, the remaining Strikers batsmen struggled to find rhythm, as Fernando (2-38) and Azmatullah Omarzai (2-31) did most damage for the Kings, with Allen getting in on the act.
Meanwhile, the Kings run chase started shakily, as the lost openers Pathum Nissanka (six) and Kusal Mendis (five), with the score at 30-2, before the Rossouw show.
The aggressive left-hander found a useful partner in Avishka Fernando, who made 58 off 35 balls, including seven fours and two sixes. The two put together a record third-wicket partnership of 120 runs, which surpassed the franchise’s previous best of 110 between Shoaib Malik and Avishka Fernando.
Though Fernando fell in the 15th over to Shadab Khan, the damage was already done, and Rossouw pressed on to seal the win, their fourth of the campaign.
With the win, Kings assumed pole position on the league table with eight points, followed by Galle Marvels (six points), with the Strikers, Dambulla Sixers and Kandy Falcons, all on four points each.
Fernando struck 96 from 102 deliveries to see the hosts score 248-7, leaving their opponents with an uphill task in difficult conditions at the R. Premadasa Stadium.
And having struggled in the second Test with Sri Lanka's spin bowlers, India did so again as Wellalage posted figures of 5-27 to secure a landmark victory.
Sri Lanka's opening three batsmen took the fight to India early on, with Pathum Nissanka (45), Fernando and Kusal Mendis (59) leaving them 183-3 in the 37th over.
Riyan Parag took three wickets as the Lions gave their opponents a target of 249 to draw the series, a task they found difficult from the beginning.
Captain Rohit Sharma (35) and Shubman Gill (six) fell early, with the Men in Blue's batting line-up crumbling to 82-6 by the 12th over.
Washington Sundar's 30 from 25 deliveries offered hope of a comeback, but Wellalage confirmed the triumph in just 26.1 overs with the wicket of Kuldeep Yadav to end India's 13-game unbeaten ODI run.
"We are in a really good mood right now, especially our coach," said Sri Lanka captain Charith Asalanka.
"He made a few changes in the team, and the boys really enjoyed the team environment."
Data Debrief: Sri Lanka end India ODI hoodoo
Sri Lanka's last bilateral ODI series victory over India was in August 1997, with India unbeaten in the next 13 such series, with 11 wins and two drawn series.
The win marked the Lions' first limited-overs series under new captain Asalanka and interim head coach Sanath Jayasuriya after Chris Silverwood's departure.
