Halep came back from a set and a break down to beat Stephens 3-6, 6-4, 6-1 and win the championship in a match made up of long points and key momentum swings.
Halep, who assured herself of retaining the No. 1 ranking with the victory, earned a fourth chance to win her first major title.
"First of all, thank you guys".
"I'm really happy it's happened in Roland-Garros in Paris - my special city". I'm more relaxed about this situation", Halep said, sounding convinced of her words as they hung in the air, until she added this: "But you never know.
"No matter how hard the adversity that you go through, there is always light at the end of the tunnel, and I'm glad she finally got her light", added Stephens, who recovered from foot surgery a year ago to win the US Open.
Halep's victory was one of the most popular for many years at Roland Garros and at times during the match, the support for the 26-year-old was deafening.
Halep won 15 of the next 18 points on Saturday, and then 12 of the next 15 games with both strength and steadiness, and she did it against a big-point player who stood 6-0 in WTA Tour finals previously.
US Open champion Stephens used her powerful game to full effect to take the first set but then ran out of steam as Romania's Halep completed a memorable victory. Claimed her first Grand Slam title at Roland Garros defeating Sloane Stephens.
Halep served out to finally snag a well-deserved first Grand Slam title.
"Congratulations to Simona. There is no-one else I'd rather lose to than the number one in the world".
When Halep ended a 14-stroke point by pushing a backhand wide, Stephens owned the first set. Stephens has dropped just one set this entire tournament and has been playing extremely clean tennis, making very few mistakes. Tenth seed Stephens, cool as a cucumber, won the next three points to take the opening set.
A string of defeats followed that fantastic feat but she kept focused until defying the odds by going all the way to Saturday's final in Paris. Not much after that, Stephens broke to begin the second set, then held for a 2-0 lead.
Halep promises to offer a more stern challenge, having won five of her seven meetings with Stephens, but the American will hope the world No. 1 will again falter in the final of a Slam, a stage on which she has so far failed to produce her best. The matchup was the first All-American semifinal match at Roland Garros since 2002. She wheeled toward her box, which included USA national football team player Jozy Altidore, and shook a fist. Serena Williams is obviously going to be the biggest star in American tennis until her career is over.
There couldn't have been a more popular champion as the tennis world celebrated nearly more than Halep did.