Roberson, 25, preferred a three-year deal over the four years available to him, league sources said.
Roberson, an NBA all-defensive second-team choice this year, has been an integral part of the Thunder's defensive identity. His value to the franchise grows again with the arrival of All-Star Paul George to partner with MVP Russell Westbrook.
Roberson averaged a career-high 6.6 points and 5.1 rebounds in 79 games this past season. He holds career averages of 4.5 points and 3.9 rebounds in 256 games.
The deal is yet another move in a productive offseason for Thunder general manager Sam Presti, who traded with the Indiana Pacers for George and made a strategic free-agent agreement with power forward Patrick Patterson for $16.4 million over three years.
Roberson has started 230 games for the Thunder in his four pro seasons, including a career-high 79 in 2016-17. He was the 26th overall pick for the Thunder in the 2013 draft.
Roberson's market was a tough one to gauge in the NBA because of his specialist role as a defensive stopper. After turning down an extension in October, Roberson didn't garner the kind of interest he expected this offseason. His offensive issues are well-known, highlighted by his struggles at the free throw line during this past postseason, when he hit only 3 of 21 attempts and was a target of intentional fouling.
But with the Thunder adding George, which will allow Roberson to slot back into his more natural position of shooting guard, there's optimism that Roberson's offensive deficiencies won't be as glaring with the enhanced supporting cast. Roberson attempted 184 3-pointers last season, hitting only 24.5 percent, but the feeling from inside the team is he'll attempt far fewer and focus more on his strengths, such as cutting and scoring in transition.
The Thunder value Roberson, taking pride in the scouting done to convert the 6-foot-7 player from a college power forward at Colorado to an NBA shooting guard. Both sides were motivated to complete a deal to keep Roberson in Oklahoma City.