Smoltz, the 1996 Cy Young winner and the only Major League pitcher with at least 200 wins and 150 saves, underwent arthroscopic surgery to repair a shoulder injury last June after posting a 3-2 record with a 2.57 ERA in just five starts for the Atlanta Braves.
Terms of the contract were not announced but MLB.com indicated Smoltz would earn $5.5 million next season plus incentives.
"I'm doing great and I have, over the last five surgeries, been, I guess you could say, a quick healer and very determined to rehab," said Smoltz, who likely will not get his first start for the Red Sox until June.
Smoltz, 41, has a career regular-season record of 210-147, and helped the Braves to National League pennants in 1992, 1992, 1996 and 1999 and a World Series championship in 1995.
Notable deaths of 2014 [PHOTOS]