Knizner agreed to a minor-league contract with the Red Sox on Tuesday, Chris Cotillo of MassLive.com reports.
After failing to crack the Mariners' Opening Day roster this spring, Knizner joined the Rockies on a minor-league deal and had a .961 OPS with 10 homers in 33 games with Triple-A Albuquerque before being released last week. The 31-year-old doesn't have a clear path to joining the big-league club since Boston already has three catchers on the 26-man roster.