The 24-year-old Ukrainian triumphed 3-6, 6-2, 6-2 in two hours and 23 minutes and became the the first player to go through the WTA finals undefeated since Serena Williams in 2013.
Svitolina, the sixth seed, beat the defending champion Caroline Wozniacki, Petra Kvitova and Karolina Pliskova at the group stage. She then outlasted Kiki Bertens in the semi-finals.
"It's a very special moment for me,” said Svitolina. “I played great tennis this week and this is going to bring a lot of confidence as I have finished the season on a high note."
Stephens said she was satisfied with her first appearance in the WTA finals. "Not the way I wanted to finish but I worked really hard to get here and I can be really proud of my results," she said.