Stephens joins fellow Grand Slam champions Simona Halep, Angelique Kerber, Naomi Osaka, Petra Kvitova and Caroline Wozniacki in the singles field for the Oct. 21-28 event.
Two spots remain open for the elite eight-player field, with Elina Svitolina and Karolina Pliskova in prime position to qualify for the round-robin tournament that offers $7 million (5.34 million pounds) in prize money.
"I'm honoured to qualify for my first WTA Finals," said Stephens, who was runner-up to Halep at Roland Garros earlier this year. "I'm looking forward to ... competing against the best players of the season."
The American also won her first Premier-level event in Miami earlier this year and rose to a career-high ranking of three in July. The 25-year-old was also a runner-up in Montreal.
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Stephens won her first Grand Slam title at Flushing Meadows in 2017 but was knocked out in the quarter-finals by Anastasija Sevastova this year.
Osaka won the U.S. Open, while Kerber won Wimbledon and Wozniacki clinched the Australian Open title this year.
(Reporting by Greg Stutchbury in Wellington; Editing by Peter Rutherford)
