The Flint, Mich., native is having a breakout season as Boston's starting goalie. Entering play Thursday night, Thomas was 32-11-7 for the East-leading Bruins. Thomas leads the league in goals-against (2.13) and save percentage (93.1) and made his first NHL All-Star appearance in February.
Boston is expected to announce the deal Saturday.
Thomas was drafted 217th overall by the Quebec Nordiques in the 1994 NHL draft after his freshman year at the University of Vermont. Prior to the 1997-98 season he opted to play in Europe. Thomas bounced between Europe, the American Hockey League, the International Hockey League and the NHL until joining the Bruins full-time in 2005-06.
Thomas became the starter last year after an early-season injury to Manny Fernandez. Thomas finished the year 28-19-6 with a GAA of 2.44 and save percentage of 92.1.
Thomas was scheduled to be an unrestricted free agent after this season. Terms of the deal were unavailable.
ESPN's John Buccigross contributed to this report.