Ukrainian Svitolina had arrived in Singapore woefully out of form, but the baseliner gathered momentum as the tournament progressed, winning all three pool matches and semifinal before subduing a fast-starting Stephens.
Her victory over Stephens was her ninth straight finals win and 13th from 15 overall.
With her arms around the Billie Jean King Trophy, Elina Svitolina wraps up the fifth and final edition of the BNP Paribas WTA Finals Singapore presented by SC Global.
Svitolina jumped to a 3-1 lead in the second set, only to concede her own serve in the very next game to narrow the gap to 3-2.
Stephens' blistering forehand dictated proceedings, as she ran away with the first set.
The Ukrainian beat her American opponent 3-6 6-2 6-2 in two hours and 22 minutes for the biggest victory of her career. "This is going to bring a lot of confidence, as I have finished the season on a high note".
As WTA Singapore tournament is over, the victor Elina Svitolina shared her impressions with the press.
"I think I have nothing to prove anymore to anyone and it's definitely a good statement for myself", Svitolina said.
Both Stephens and Svitolina are unbeaten in the competition, having won all three round-robin games and whoever emerges champion later today, will be a maiden victor at the season-ending WTA Finals.
Earlier, Timea Babos and Kristina Mladenovic played a masterclass to upset world number one Barbora Krejcikova and Katerina Siniakova in the doubles decider.
Elina Svitolina on the other hand, edged Kiki Bertens 7-5, 6-7 (5-7), 6-4 to make the final.
"The first tournament of the season, we won a slam (Australian Open) and the last tournament of the season we won.it's a privilege", Mladenovic said after the match.
And the world number seven continued her dominance into the final set, breaking three times once more to secure the title.