UFC president Dana White broke the news to ESPN on Tuesday, and Cormier confirmed it to ESPN. The heavyweight fight will take place on the UFC's annual card at Madison Square Garden.
With a new main event in place, Valentina Shevchenko will be moved back to her original fight against Joanna Jedrzejczyk for the vacant women's flyweight title at UFC 231 on Dec. 8 in Toronto, according to White.
The UFC had canceled that fight last week in order to have Shevchenko headline UFC 230 against Sijara Eubanks. But with the Cormier-Lewis title bout set as the new headliner, the promotion decided to move Shevchenko back to the Dec. 8 card.
Cormier (21-1) is the UFC's light heavyweight and heavyweight champion. He is also the No. 1 pound-for-pound fighter in the world, according to ESPN's rankings.
His next heavyweight title bout was expected to come against Brock Lesnar in 2019, but the UFC wanted to beef up next month's pay-per-view event, and Cormier had expressed a willingness to fight one more time this year. He is coming off a historic knockout victory over Stipe Miocic at UFC 226 in July.
Cormier, who is based out of San Jose, California, has said he will retire next spring, around his 40th birthday.
Lewis (21-5), of Houston, is coming off a stunning come-from-behind knockout victory over Alexander Volkov at UFC 229 last weekend in Las Vegas. Lewis was trailing in the fight until the final 20 seconds, when he dropped Volkov with a right hand. He has won nine of his past 10 bouts.