Halep won the match 3-6, 6-4, 6-1 on Court Philippe Chatrier.
"It's a special moment," Halep told reporters. "I was dreaming for this moment since actually before I started to play tennis."
"It's my favorite Grand Slam. I always said that if I am going to win one, I want it to be here. It's real now."
Stephens spoke about Halep's rally after the first set.
"She raised her game," Stephens said. "She raised her level. There is not much you can really do about that. I competed the best that I could and the better player won the match today."
Stephens, 25, won the 2017 U.S. Open for her first career Grand Slam. Halep, 26, recently lost to Caroline Wozniacki in the 2018 Australian Open, her first time appearing in a Grand Slam final before Saturday's showdown.
Halep can appear in a third straight Grand Slam final if she reaches the finale of Wimbledon, which begins on July 2 in London. The No. 1 tennis player in the world beat Garbine Muguruza in the semifinals and Angelique Kerber in the quarterfinals at the French Open.