The 27-year-old prevailed 7-6 (7-4) 6-0 in Seoul to add to previous titles won in Shenzhen and 's-Hertogenbosch, the latter coming just three months ago.
Alexandrova is at a career-high 24th in the WTA rankings and is hoping to climb further before the year is out.
"You always want to achieve more. I want to go higher and maybe end the year in the top 20 in the best-case scenario," she said in her on-court interview.
Ostapenko advanced to Sunday's final after Emma Raducanu retired injured in the previous round and was aiming to become the first two-time champion in the event.
Second seed Alexandrova dug deep in the first set by recovering from a break down three times against the 2017 winner before saving a set point at 6-5 down.
The Russian looked far stronger in the second set as she posted her sixth bagel of 2022 to seal victory in a time of 85 minutes.
"I think it was tough for me to start because I was really nervous and I couldn't find my rhythm in the first couple of games," Alexandrova said.
"But the longer we played, the better I felt on the court. I think maybe somewhere in the end of the first set I finally found the rhythm and I just continued to keep it that way."
Badosa was defeated in just 78 minutes by Zheng Qinwen, with the number one seed going down 6-3 6-2.
It was 19-year-old Zheng's first completed win against a top 10 opponent in her career, and she said afterwards: "Even though the score was like this, there were a lot of emotions for me in this match and I'm very proud of myself. It's not a surprise. I always knew I had the level, I just had to make it."
Second seed Garcia hit 27 aces in her match against Zhang Shuai, the most in a single WTA-level contest since Kristyna Pliskova against Monica Puig in Luxembourg 2019 (28), but it was not enough as she lost two tie-breaks to lose 4-6 7-6 (7-5) 7-6 (7-5).
Number three seed Garbine Muguruza had more success as she beat Greek opponent Despina Papamichail 6-4 6-2, while Claire Liu also eased to a win against Elise Mertens 6-4 6-1, and will play Zheng in the quarter-finals.
Eighth seed Elena Rybakina suffered a first-round exit as she lost to Liudmila Samsonova 6-2 6-4, who will play Xinyu next.
The seeds fared better over at the Korea Open, with Emma Raducanu sealing a first-round win against Moyuka Uchijima 6-2 6-4, while Magda Linette also eased past Arianne Hartono, 6-2 7-5, though fifth-seed Varvara Gracheva was knocked out by Anna Blinkova 6-4 7-6 (7-1).
In the second round, third seed Ekaterina Alexandrova is through after beating Han Na-lae 6-1 6-3, as is Lin Zhu of China following her straightforward 6-1 6-3 victory against India's Ankita Raina.
There were also wins for Tatjana Maria against Kimberly Birrell, and 17-year-old Andorran Victoria Jimenez Kasintseva against eighth seed Rebecca Marino.
American Kovacevic was only playing in the first round as a lucky loser in Seoul after he was beaten in the final round of qualifying, but he grasped his chance by winning 6-4 6-4.
The 24-year-old served 14 aces and broke the world number 32 three times to set up a second-round meeting with Christopher O'Connell or Tseng Chun-hsin.
Radu Albot got the better of Hiroki Moriya in his first appearance in Seoul, while Jaume Munar progressed when Yosuke Watanuki retired early in their match due to an ankle injury.
Kaichi Uchida came out on top in the final match of the day, consigning Hong Seong-chan to a 6-4 6-2 defeat.
The Japanese player earned a 6-4 7-6 (7-5) triumph in a competitive contest against fourth seed Shapovalov on Sunday that lasted just under two hours.
Victory gave Nishioka his first trophy since the Shenzhen Open in 2018 and capped a memorable week that also saw him defeat top seed Casper Ruud at the ATP 250 event in Seoul.
Nishioka took a big step towards the second title of his career by breaking Shapovalov at a crucial moment when he was 5-4 up in the opener, both players having earlier saved break points.
His Canadian opponent was in the ascendancy after claiming an early break in the second set but Nishioka fought back from 3-1 down to force a tie-break and then won four of the first five points in the breaker.
Despite Shapovalov reducing the deficit, he was consigned to defeat when he sent a forehand wide, Nishioka dropping to his knees in delight after converting his second match point.
Shapovalov, a Wimbledon semi-finalist last year, has now lost in four consecutive ATP final appearances and still has just one career title to his name.
Nishioka, who will return to the ATP's top 50, eliminated fifth seed Dan Evans in the first round and later top seed Casper Ruud in the quarter-finals.
Meanwhile, Kovacevic made his main-draw bow on Monday, beating seventh seed Miomir Kecmanovic, and continued a sensational run all the way to the last four.
The American threatened another upset on Saturday, but Nishioka held his nerve to advance 6-3 4-6 6-3 in Seoul.
And now the Japanese will revert to the role of underdog on Sunday, with fourth seed Denis Shapovalov his opponent in the championship match.
Nishioka has only one ATP Tour title to his name, and he will have his work cut out if he is to add to that against Shapovalov, who is yet to drop a set this week.
The Canadian continued that sequence with a 7-5 6-4 success against Jenson Brooksby in the first semi-final.
Nishioka moved into the second round in Seoul with a 6-2 7-6 (7-3) defeat of the Briton.
The world number 56 from Japan will face Taro Daniel or Emilio Gomez in round two after winning 70 per cent of points on his first serve and breaking Evans four times.
Jenson Brooksby was forced to go the distance, but delivered a final-set bagel as the eighth seed saw off Wu Tung-lin 7-6 (7-5) 3-6 6-0.
Qualifier Nicolas Jarry will take on top seed Casper Ruud after defeating Ryan Peniston 6-3 7-6 (10-8), while American duo Mackenzie McDonald and Steve Johnson also advanced.
Tseng Chun-hsin progressed with Chris O'Connell retired with an injury after losing the first set before Kwon Soon-woo beat Yunseong Chung in an all-South Korean contest.
Four-time grand slam champion Osaka won the opening game, before Saville was unable to continue due to a scary-looking injury to her left knee.
The former world number one will now face a second round match with world number 16 Beatriz Haddad Maia, who eased to a straight sets victory over Yuki Naito.
Karolina Pliskova secured her place in the next round in Tokyo with a 6-2 6-1 demolition of Isabella Shinikova, while Zhang Shuai will face second seed Caroline Garcia on Wednesday after overcoming Mai Hontama in two sets.
In Seoul, Jelena Ostapenko's hopes of repeating her 2017 heroics at the Korea Open are still alive after the number one seed narrowly beat 19-year-old Jeong Bo-young.
The world number 19 looked to be cruising after the opening set, but the unheralded South Korean hit back in the second and took it to a tie-break in the third, before Ostapenko's class showed as she won the crucial tie-break 7-2.
Elsewhere, 2021's beaten finalist Kristina Mladenovic saw off wildcard Park So-hyun 6-4 4-6 6-3, while second seed Ekaterina Alexandrova overcame a worrying second set to beat Asia Muhammad 6-1 3-6 6-1.
Wimbledon semi-finalist Tatjana Maria is also safely through to the second round after a tight 7-6 (7-5) 7-6 (7-5) triumph over Eugenie Bouchard.
The former world number one, who won the Tokyo tournament when it was last held in 2019, was unable to compete in Thursday's match due to illness.
"I am really sorry that I am not able to compete today," Osaka said. "It's an honour to be able to play at the Pan Pacific Open in front of the amazing fans here in Japan.
"This has and always will be a special tournament for me and I wish I could have stepped on court today, but my body won't let me."
Haddad Maia advanced on a walkover and will next face fourth seed Veronika Kudermetova, who defeated Mexican qualifier Fernanda Contreras Gomez 6-0 6-1 to reach the quarter-finals.
Liudmila Samsonova also progressed to the last eight, with a 7-6 (7-5) 6-3 win over Wang Xinyu continuing her good form.
However, sixth seed Karolina Pliskova exited the competition on Thursday at the hands of Petra Martic, who prevailed 6-3 6-4 and will face Zhang Shuai next.
At the Korea Open, top seed Jelena Ostapenko recovered from two games down in the deciding set to overcome Anastasia Gasanova 6-3 5-7 7-5.
Ostapenko remains on a possible semi-final collision course with Emma Raducanu, who came through 6-4 7-5 against Yanina Wickmayer, while Magda Linette saved two match points to beat Kristina Mladenovic 4-6 7-6 (7-5) 6-2.
Raducanu required just 77 minutes to see off the third seed for a 6-2 6-2 win, dominating from the outset to clinch her first final four spot in any tournament since last year's famous US Open triumph.
Having wrapped up the opener with a fierce backhand winner, Raducanu was forced to endure a brief revival from Linette in the second set, before the Pole was troubled by a thigh injury.
Linette lost 14 of the first 17 points on resumption after a medical time out, struggling with her movement as Raducanu saw out the match.
Raducanu has yet to drop a set during her run in Seoul, but will face a stern test in the semi-finals after Ostapenko overcame 17-year-old Victoria Jimenez Kasintseva 6-2 6-1, setting up a first career meeting between the duo.
On the other side of the draw, Ekaterina Alexandrova earned a hard-fought 7-5 7-6 (8-6) win over Lulu Radovcic, but fourth seed Zhu Lin crashed out to Tatjana Maria, losing 6-1 6-1.
In the Pan Pacific Open in Tokyo, meanwhile, Garbine Muguruza fell to a shock 6-4 6-2 quarter-final defeat to Liudmila Samsonova, handing the Russian her 16th win in her last 17 outings.
She will meet Zhang Shuai in the next round, after she became the first Chinese player to make the last four in Tokyo since 2009 by beating Petra Martic 7-5 6-2.
In a battle between the competition's fourth and fifth seeds, meanwhile, Veronika Kudermetova rallied to beat Brazil's Beatriz Haddad Maia 6-7 (4-7) 7-6 (8-6) 6-1.
The 2021 US Open winner was featuring in her first semi-final since that fairytale triumph at Flushing Meadows, and took the opening set 6-4 after making a flying start on Saturday.
Ostapenko then fought back to level the match, with Raducanu first appearing to struggle with her movement when she was a break up in the second, forcing her to take an off-court medical timeout.
The 19-year old later retired citing a left glute injury when 3-0 down in the decider, ending her confidence-boosting run in Seoul and teeing up a final meeting between the top two seeds.
Indeed, Alexandrova enjoyed a more routine outing in her meeting with Wimbledon semi-finalist Tatjana Maria, downing the German 6-2 6-4 to advance to her second final of the year.
At the Pan Pacific Open in Tokyo, meanwhile, Chinese teenager Zheng Qinwen reached her first WTA Tour final with a hard-fought win over world number 13 Veronika Kudermetova.
Having required over three hours to seal her 5-7 6-3 7-6 (7-3) triumph, the world number 36 said: "After the match when I won the last point, the feeling was incredible.
"The opponent, she's playing unbelievable tennis. She has a really good serve and hits the ball hard, she moves well. It's my first time in the final. I'm so happy."
Her compatriot Zhang Shuai was unable to replicate those exploits, however, as Liudmila Samsonova eased into her third WTA final of the year with a 7-6 (7-4) 6-2 win.
In-form Samsonova has won both of her previous final appearances this year - each of which came last month - and is 3-0 in showpiece matches for her career.
Riske-Amritraj reached the last 16 at the US Open earlier this month but has struggled in Asia since then.
She was the top seed at the Chennai Open last week, only to lose in straight sets in the first round to Anastasia Gasanova.
And the American fell at the first hurdle again in Japan.
Riske-Amritraj was drawn against compatriot Claire Liu, who was coming off back-to-back defeats to Anastasia Potapova and without a win since qualifying for the Canadian Open last month.
But Liu brushed aside the seventh seed 6-2 6-3 to set up a meeting with Elise Mertens in the second round.
Mertens eased through with a 6-0 6-3 win over Wang Qiang, while Petra Martic was similarly comfortable in a 6-1 6-1 defeat of Rina Saigo.
Saigo was one of two home hopefuls to depart on Monday as Qinwen Zheng won 19 points in a row en route to beating Misaki Doi 6-2 6-4.
At the Korean Open, Zhu Lin was the only seed in action and got the better of Jang Su-jeong in straight sets.
The top seed was given a second set scare by the Chilean but wrestled back control to emerge as the victor in a 6-2 3-6 6-3 encounter.
The two-time grand slam finalist, having been runner-up at Wimbledon and the US Open this year, will face Yoshihito Nishioka after he saw off Japanese compatriot Taro Daniel in a 6-2 6-4 win.
His victory also ensured his place at the ATP Tour's end-of-year finale in Turin, alongside Spanish duo Rafael Nadal and Carlos Alcaraz.
Cameron Norrie remains on course to set up a potential final in Korea with Ruud after brushing aside Kaichi Uchida with a comfortable 6-2 6-2 victory.
The Briton will face eighth seed Jenson Brooksby after he turned over home favourite Kwon Soon-woo in a 6-3 6-4 result in Seoul.
Brooksby's fellow Americans Mackenzie McDonald and Aleksandar Kovacevic also came through their last-16 encounters to set up a quarter-final clash against each other.
The 2021 Wimbledon semi-finalist needed 109 minutes to claim a 7-5 6-4 win over Munar, assuming control by breaking in the seventh game of the opener before doing likewise early in the second set.
Canadian Shapovalov – who is seeded fourth for the event – had not featured since a dramatic loss to Andrey Rublev at the US Open earlier this month, but produced a consistent display in Seoul, not committing a single double fault during the match.
His reward for the victory is a last-eight tie with Radu Albot, who held his nerve in two tie-breaks to record a 7-6 (7-3) 7-6 (7-3) win over Steve Johnson.
In Wednesday's other match, Japan's Taro Daniel beat Ecuador's Emilio Gomez in straight sets in the duo's round-of-32 tie.
Top seed Casper Ruud begins his campaign against Nicolas Jarry on Thursday.
