Alfaro signed a one-year contract with the Red Sox on Thursday, Alex Speier of The Boston Globe reports.
Alfaro caught on with the Rockies after opting out of his minor-league deal with the Red Sox last month, but he's now back with Boston after Colorado cut him loose earlier this week. The 30-year-old will back up Connor Wong at catcher while Reese McGuire (oblique) is sidelined and might also see a little action in the outfield and designated hitter.