Krejci, 28, led the Bruins last season with 69 points, including 19 goals, in 80 games. His career highs for a season are 23 goals (in 2011-12) and 73 points (2008-09).
The Czech led all NHL players in plus/minus last season with a plus-39 rating. Teammates Patrice Bergeron and Brad Marchand were second and third, respectively.
Krecji helped Boston win the Stanley Cup in 2011, when he topped all post-season scorers with 23 points in 25 games.
His new deal begins with the 2015-16 season and runs through 2020-21.
Krejci will earn $5.25 million this season in the final year of his current contract.