The deal includes a player option on the fourth year, the sources said.
Without taking any free-agency meetings, George made the announcement that he is staying with the Thunder at a house party in Oklahoma City hosted by Russell Westbrook, sources at the party confirmed.
George took to Instagram late Saturday to post a photo of himself with Westbrook, along with the caption, "Unfinished business."
With many assuming that the Los Angeles Lakers were the unquestioned favorites for George, his decision to stay with the Thunder came as a bit of a shock. However, sources told ESPN's Ramona Shelburne that George informed the Thunder of his intentions well before Saturday's party so they could operate in free agency with confidence that he was staying.
George was traded to OKC almost exactly a year ago after informing the Indiana Pacers that he would not re-sign with them, instead preferring to join his hometown Lakers in free agency. The Thunder spent the year recruiting George to sign up long-term, beginning with fans welcoming him at the airport and a welcome party organized a day after.
"I haven't been in a position to be a free agent or know what that is like and then to have the chance to go where you want to go. That's always been, I feel with players, they want that option," George said at exit interviews after the season. "But then you go into an organization where they kind of check the boxes on all the things that you want out of an organization, and then immediately they become a candidate to where you want to play long-term.
"So that was the case. That's been the case here," he said. "They honestly check the boxes where I needed those boxes to be checked from what a player wants and needs out of a front office, out of a medical group, out of teammates, out of coaching staff. I mean, honestly, they can't say it anymore [than] that. They checked the boxes on what needs to be checked."
The Thunder have been in communication with George since the end of the season and remained in contact with him throughout the past week. The signs and momentum for George staying had been building throughout the season, hinging largely on his trust of Thunder general manager Sam Presti and his strong relationship with Westbrook. George talked often about "building" with the Thunder, referencing his enjoyment of playing with Westbrook.
"Russ is the reason why this decision is becoming even more easier to make, is the character Russ [has]," George told ESPN's Rachel Nichols in February. "A stand-up guy, and he has his teammate's back."
After acquiring Carmelo Anthony in September, the Thunder appeared to have formed a superteam with the then-reigning MVP. However, the season didn't go as planned, with a bumpy start finally smoothed out by January before an injury to starting shooting guard Andre Roberson derailed it again. The Thunder recovered to finish with 48 wins and the Western Conference's 4-seed, but they went out in six games to the Jazz in the first round. In George's final game, one in which Westbrook scored 46 points on 43 shots, he finished with just five points on 2-of-16 shooting.
"Here they have made a huge risk in trading for me, knowing I have one year on my deal," George said in part one of SportsCenter's behind-the-scenes look at his free agency. "But I felt I didn't finish as strong as I could have. Just knowing you left something on the table, even to this point now, it weighs on me."
George, a five-time All-Star, hit unrestricted free agency for the first time in his career after declining his player option last Thursday.
With George re-signed and Carmelo Anthony under contract for next season, the Thunder are headed for a monumental payroll and luxury-tax bill, something ownership is prepared to pay. The Thunder could take measures to reduce their financial stress by using the stretch provision to waive Anthony or negotiating a buyout or trade (Anthony has a no-trade clause, though). The Thunder also are interested in re-signing forward Jerami Grant.
George, 28, averaged 21.9 points on 43.0 percent shooting plus 5.7 rebounds and 3.3 assists. Playing with Westbrook, George enjoyed one of his best 3-point shooting seasons to date, hitting the highest total of his career on 40.1 percent shooting.