Team executive vice president and general manager Sam Presti announced the deal Thursday night, but didn't disclose any terms.
Durant's spokeswoman, Mary Ford, said Wednesday that Durant would receive the maximum deal possible, $85 million over five years.
Durant was the No. 2 overall pick in the 2007 draft and won the Rookie of the Year award. He averaged 30.1 points and 7.6 rebounds while leading the Thunder to a 50-32 record -- a 27-win improvement over the previous season -- and the first round of the NBA playoffs.
Presti says the signing shows Durant is committed to the team and the team is committed to Oklahoma City and the state.