Ostapenko, playing in only her eighth Grand Slam tournament, had never before won a WTA title.
Not only that but she is the first unseeded woman to win the Suzanne Lenglen trophy since 1933 and she emulated the great Brazilian Gustavo Kuerten by claiming her first title of any kind on the hallowed Parisian claycourts.
It was a second French Open final defeat for Halep, who also lost to Maria Sharapova in 2014.
Ostapenko's carefree attacking tennis ultimately proved too strong for the consistency and experience of Halep, with the favourite helpless by the closing stages.
As usual, Ostapenko took all the risks, and had the same number of winners and unforced errors at 54.
But Ostapenko then got broken just as rapidly in the next game.
Nevertheless, it was not until the final game, with Ostapenko serving to stay in the set at 4-5, that Halep was able to break her again. She must have hoped that Ostapenko would freeze on the big occasion but instead the Latvian looked relatively relaxed.
She stormed through the last five games in a row, apparently free of nerves, to clinch victory and grab her place among the elite of women's tennis.
Once again, the Latvian continued to hit the ball hard, producing more scintillating backhand winners down the line, and she broke Halep's serve again.
Ostapenko said she was more than comfortable with dealing with her sudden fame which will be under a harsh spotlight when Wimbledon starts in four weeks' time. "That's why I work 20 years and played 20 years to have this moment".
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With 1:24 to play in the fourth, and with nearly everything breaking right in the second half, the Cavs were up four. TURNING POINT: This one was in doubt every step of the way, as neither team was able to build a double-digit lead.
"I think it was a good tournament". I want to have many more, if it's possible.
"I mean I can't believe I'm champion at 20", Ostapenko, the youngest finalist for 10 years, said on court. When I was a child I was watching. It's so incredible to be here. "I'm really happy. I have no words, it was my dream". But Halep, 25, said she felt sick to her stomach in the previous 24 hours leading up to the final as she tried, in vain, to get the enormity of the occasion out of her mind.
"At some point I was like a spectator on court", said Halep.
STAT OF THE DAY: Ostapenko launched an impressive tally of 54 winners in the final to take her tally to 299 for the tournament.
"Of course, there will be more pressure but I will try and deal with it. Enjoy it be happy and keep it going because you're like a kid", she said. I thank you for coming to support me always.
Nadal has a 15-3 record against Wawrinka, but the third-seeded Swiss player won the only time they met in a Grand Slam final, at the 2014 Australian Open.
"Ostapenko was a teenager before Thursday and had never previously featured in the second week of a Grand Slam".
Halep consolidated with a gritty hold for 3-0 before Ostapenko fought off a trio of break points to stall her rival's momentum and spark an incredible comeback.
That's because from about the age of 5, Ostapenko juggled her two hobbies, dancing and tennis, until picking one at 12.
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