The deal includes a club option for a seventh year that could bring the overall value of the contract to $63 million, the source said. Bruce has a partial no-trade clause and the opportunity to earn additional money through award bonuses.
Bruce, 23, is regarded as one of baseball's budding young stars. He hit .281 with 25 homers and 70 RBIs last season to help lead the Reds to a National League Central Division title. He had a .575 slugging percentage and a .951 OPS after the All-Star break.
Bruce would have been eligible for salary arbitration in February as a "Super Two'' category service time player.
The Reds signed pitcher Bronson Arroyo to a three-year, $35 million contract extension a week ago. The team has also expressed interest in negotiating a multiyear contract with first baseman Joey Votto, the 2010 National League MVP.
Jerry Crasnick is a senior baseball writer for ESPN.com.