The 20-year-veteran finalized a 1-year contract worth more than $5 million but the deal could be worth more than $10 million after performance bonuses.
The 41-year-old Smoltz brings 210 career wins, 154 saves and a 3.26 ERA to the Red Sox.
"I'm as determined and I'm as focused as I've ever been," Smoltz said Tuesday in a news conference.
Smoltz is coming off shoulder surgery and no timetable has been set for his return, but both he and General Manager Theo Epstein said he'll be a starter once he's ready to go.
With Boston's strong rotation, which includes Josh Beckett, Jon Lester, Daisuke Matsuzaka and the newly acquired Brad Penny, the team can bring him along slowly.
Smoltz, who had spent his entire career in Atlanta, said he has no bitterness toward the Braves.
"They were taking a different direction and, for the most part, left me with really no options," Smoltz said. With the deal, Smoltz becomes just the 8th player in history to spend at least 20 seasons with one team, before signing with another.
In news of former Sox players, Atlanta Braves is closing in on a $60 million, four-year deal with Derek Lowe, according to a person familiar with the negotiations.
Lowe was a 21-game winner for the Red Sox in 2002 and spent the last four seasons in Los Angeles, where he went 54-48, never had an ERA higher than 3.88 and averaged more than 200 innings a season.