Pliskova would have reached a second Grand Slam final and faced Ostapenko on Saturday for the French Open title - plus jumped to No. 1 in the rankings next week - but she lost to Halep in the semifinals.
Not since Mima Jausovec in 1983 has a player outside the leading ranks made it all the way to the decider.
"I'm really happy with my win today and it was really tough match".
The third seed from Romania said: "I will play for two things: My first grand slam and number one in the world". I think it's a nice gift!
"I was always playing aggressive and hitting the ball when I had a chance".
Pliskova would have risen to No.1 had she defeated Halep, but she fell to the 25-year-old for the fifth time in six career meetings despite mounting a late rally to force a decider.
- Ostapenko also praised her coach Anabel Medina Garrigues, a former world number 16, who teamed up with the Latvian earlier this season. "I'm really happy to be in the final, especially on my birthday. I take on a young player, it will be an interesting challenge".
The unseeded Ostapenko became the first Latvian to reach a major final with a 7-6 (7-4) 3-6 6-3 victory on Court Philippe Chatrier.
Ostapenko said: "In some moments I felt a little bit tight because it was already the semi-final".
Just last month, Halep lost to Svitolina who leads the tour in titles and match wins in 2017 in the final of the Italian Open.
"Third set everything was through the long rally, which I think is better for her and she's winning almost - not all of them - but a higher percentage, definitely", said Pliskova.
Should Ostapenko, who had never won a match at the French Open before last week, beat Halep she would become only the second unseeded player to win the women's title after Great Britain's Margaret Scriven achieved the feat in 1933.
A scruffy game followed from Ostapenko as a huge opportunity appeared to go begging, though she made amends in the tie-break.
"But then, like, first match was a tough match, as well, and I won it".
- Bacsinszky took a medical timeout to receive treatment on her right leg while trailing 4-3. "During one of the rain delays (on Tuesday) she said "I'm young, I have nothing to lose, I have a lot of Roland Garros to play so I'm going to play free" and that's a very good mentality that she thinks that and just goes on the court and just plays free".
Ostapenko extended a history-making assault in Paris, becoming the first Latvian in the professional era to reach a grand slam final. "That gap won't be there at Wimbledon because (Maria) Sharapova will come back, Victoria (Azarenka) will come back, (Petra) Kvitova will be better than ever", Evert said.
It was clear from the start that she was going to be the aggressor and she edged a first set in which she hit 21 winners.
Ostapenko, whose average forehand speed of 76 miles per hour this tournament has eclipsed that of Andy Murray (73), continued to let fly with a bombardment of winners and errors.