Santos, 38, will make his PFL debut in 2023 when he competes in the promotion's season format with a tournament championship purse of $1 million. Santos will fight in the 205-pound division.
"We are very excited to bring in Thiago Santos to the PFL light heavyweight division," Sefo told ESPN. "He is known throughout the MMA world as a guy who delivers exciting fights, and I'm looking forward to seeing his elite knockout power up close."
Santos most recently appeared in a UFC Fight Night main event against Jamahal Hill in August, losing via fourth-round TKO. Six of his past seven appearances in UFC were in main events, including a failed title shot against then-champion Jon Jones at UFC 239 in July 2019. Santos lost that fight via split decision.
He was nearing the end of his UFC contract, and the two sides ultimately decided to part ways. Santos had fought exclusively in UFC since 2013.
"It was an amicable separation with the UFC," Santos' manager Alex Davis told ESPN. "Thiago is very grateful for everything the UFC did for him, and he's very grateful toward the PFL for signing with him. He's looking forward to winning a million dollars."
Fighting out of Brazil, Santos came close to dethroning Jones in their five-round title fight despite suffering a serious knee injury during the bout. He has struggled to regain his old form since the injury, suffering losses to Glover Teixeira, Aleksandar Rakic, Magomed Ankalaev and Hill. Nevertheless, he was still ranked No. 9 in UFC's light heavyweight division.