Grzelcyk, who was a restricted free agent, had a salary arbitration hearing scheduled for Tuesday.
The Massachusetts-born blue-liner was coming off a two-year contract that carried a cap hit of $1.4 million.
Grzelcyk collected a career-high 21 points and averaged 18:04 in ice time over 68 games for Boston during his third full NHL campaign this past season. The 26-year-old has spent his entire career with the Bruins since being drafted 85th overall in 2012.
Forward Jake DeBrusk is now the team's only remaining RFA.