ESPN.com reported that Smoltz officially signed his offer sheet from the Red Sox, worth a base salary of $5.5 million but including incentives that could bring the total package up to $10 million.
Smoltz had told friends a day earlier that he had made up his mind to come to the Red Sox.
Red Sox general manager Theo Epstein kept his custom of not discussing a signing before it is officially completed.
"Some of these things that we have going on, I just can't talk about," said Epstein. "Physicals remain and other issues, but I'd be happy to talk about it when they come. But a lot of exciting things are going on."
The Red Sox are expected to formally announce the signing of Smoltz and another battle-tested power righty in Brad Penny within the next few days. Penny was scheduled to have his physical in Boston on Thursday, but results were still pending.
"They're both power guys. They have great stuff," said Red Sox second baseman Dustin Pedroia, who officially received his 2008 American League Most Valuable Player Award trophy at Thursday night's Boston Baseball Writers Dinner. "They've been doing it for a while. They want to win, too. You can tell that both of them just want to win. It's a good fit and I think it's going to be great."
The fact that the 41-year-old Smoltz is leaving the Braves, the only team he's pitched for since breaking into the Major Leagues in 1988, is big news.
With the Red Sox, the righty will get a chance to pitch on a championship-caliber team with a loaded rotation that will also include Josh Beckett, Jon Lester, Daisuke Matsuzaka, Tim Wakefield and Penny.
Smoltz, coming off major surgery on his right shoulder, isn't expected to pitch until roughly June 1.
He has a career record of 210-147 with 154 saves, a 3.26 ERA and 3,011 strikeouts.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.