The Red Sox will receive a player to be named for Stewart, whom they designated for assignment last week.
In other minor moves, Boston sent infielder Danny Valencia to the Orioles in exchange for cash considerations and right-hander Sandy Rosario to the Athletics for a player to be named or cash.
After stints with the Reds (Minor Leagues only), Blue Jays, White Sox and Red Sox, the 26-year-old Stewart hopes to finally live up to some of his promise in Pittsburgh.
Scouts have seen potential in Stewart over the years, based on the fact he has been traded for Scott Rolen, Edwin Jackson and Youkilis in his career.
Following the trade to the Red Sox, Stewart spent most of his time in Boston's organization with Triple-A Pawtucket and pitched well at that level. But the right-hander was shelled in two starts for the Red Sox, going 0-2 with a 22.24 ERA and giving up 17 hits in 5 2/3 innings.
A 19th-round pick of the Twins in the 2006 Draft, Valencia was dealt in August to the Red Sox, for whom he hit .143 with one home run in 10 games.
Boston claimed Rosario off waivers from the Marlins on Oct. 17 and designated him for assignment on Nov. 20. The 27-year-old posted a 1.04 ERA and had 16 saves over 25 appearances for Miami's Triple-A New Orleans squad this past season.