Grant, 24, has become a vital young cog of the Thunder core, a young player that All-Star free agent Paul George has been enthusiastic about Oklahoma City keeping on its roster.
Keeping Grant on a $9 million annual salary could cost the Thunder an additional $40 million in luxury tax. His re-signing is a clear message to George and All-Star guard Russell Westbrook about ownership's willingness to possibly host the largest payroll in NBA history.
Grant, a 6-foot-9 forward, averaged 8.4. points on 54 percent shooting, 3.9 rebounds and one block last season. He averaged 7.8 points and 23 minutes in the Thunder's first-round playoff series loss to the Utah Jazz.
Grant was the 39th overall pick to the Philadelphia 76ers in the 2014 NBA Draft.