Ostapenko, who was celebrating her 20th birthday on the same day she beat Bacsinszky, will face the third-seeded Halep in the final after the Romanian outfoxed Pliskova 6-4, 3-6, 6-3 to reach her second singles final at Roland Garros.
However, with the final now set, Halep can turn her attention to Ostapenko, claiming the No. 1 seed and her first Grand Slam.
2014 was her best finish in a Grand Slam losing out to Maria Sharapova in the French Open Final.
Now ranked 47th in the world, Ostapenko was able to get by the 11th-seeded Caroline Wozniacki in the quarterfinals, the 23rd-seeded Samantha Stosur, the unseeded Lesia Tsurenko, world number 40 Ekaterina Makarova and American Louisa Chirico in the opening round to reach the finals.
Halep marked herself out as a favourite for the title at Roland Garros after winning in Madrid and finishing runner-up in Rome, but a tumble in the Italian Open final left her with ankle ligament damage and cast doubt over whether she would play in Paris.
Ostapenko is the first Latvian grand slam finalist and the youngest at any grand slam since Caroline Wozniacki in NY eight years ago. I have the mentality to win, but it's going to be tough.
The Romanian was under pressure early in the decider but finally chased down a forehand and fired a brilliant cross-court victor to grab a 3-1 lead.
It looked like the same might happen in the third when Bacsinszky fought back from 3-1 to 3-3 but Ostapenko had learned her lesson and clinched victory with a forehand into the corner, her 50th victor.
At times during a match of 16 service breaks the impetuosity of youth betrayed her as she seemed on the verge of youthful tantrums in the hot sunshine, especially when she lost four games in a row to lose the second set.
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Jack Hobbs , the 2-1 favourite, looked to be in contention but faded to finish last. "We knew he'd see it out really well". Scottish made most of the running, with Highland Reel hot on his heels and Decorated Knight not too far away, either.
In the end, she could at least reflect on progress in Paris - before this year, she had never got beyond the second round in five attempts.
Despite that hefty backdrop, both players played unruffled tennis and did as they often do: Pliskova served bombs and painted lines with her fearsome forehand. "I really like competing in this tournament, but it was not easy as we had to go to the locker room twice for rain delays". I came through a tough match yesterday, but I just care about today. "I was already in the semis, and I was just trying to enjoy the game and show my best".
The Latvian turned 20 on Thursday and Marion Bartoli led the Court Philippe Chatrier crowd in singing "Happy Birthday", in French, to Ostapenko moments after her victory.
Right from the off Ostapenko showed her confident best, crunching victor after winner, interspersed with unforced errors.
"This was my second time, so going forward, I think this can help me build more confidence". The two players then each held serve for the remainder of the first set before Halep was able to close out the first set 6-4.
"I don't know how I came back", admitted 25-year-old Halep, who ran away with 12 of the last 13 games to avenge her loss to Svitolina in the Rome final.
"Coming into this tournament, there were a few people who told me, "you have to be in the final to be number one".
She benefited from Pliskova's clumsiness in the first set before the Czexch rediscovered her touch.
"I am very happy. I think it's a nice gift", she said.
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