Agent Patrick Morris confirmed the signing prior to the noon start of the NHL's free-agency period.
McCormick has six-plus years of NHL experience and earned a regular role in the Sabres lineup last season. That's when he had eight goals and 12 assists for 20 points in 81 games - all career bests.
The Sabres initially signed him as a free agent in August 2009. McCormick spent the next season in the minors before appearing in three playoff games with Buffalo in 2010. He previously spent parts of five seasons with Colorado.
Overall, he has 50 points (17 goals, 33 assists) in 271 NHL games.
While McCormick returns, center Tim Connolly's days in Buffalo appear to be over after he failed to reach a deal to re-sign with the Sabres.
Connolly's agent, J.P. Barry, confirmed his client's intentions to test free agency on Friday.
Connolly had been the Sabres' longest active serving player after being acquired in a trade that sent former captain Michael Peca to the Islanders in the summer of 2001. The 11-year veteran is a play-making center who had just completed a two-year, $9 million contract.
Injuries have hampered Connolly's playing time, as he missed the entire 2003-04 season due to post-concussion symptoms. He's not played an entire NHL season since his first year in Buffalo, and had been limited to playing 98 of a potential 246 games over a three-season stretch spanning 2006-09.
Connolly had 13 goals and 29 assists for 42 points in 68 games last season.