Stewart, 26, posted a 1-4 record and 8.58 ERA in 20 outings (three starts) last season between Chicago and Boston. He went 1-2 with a 6.00 ERA for the White Sox before the trade. Stewart also made 11 starts for Triple-A Pawtucket last season, compiling a 3-5 mark and 3.94 ERA.
The Red Sox traded Stewart to Pittsburgh on Nov. 28. The Pirates designated him for assignment last Thursday.
A third-round selection by the Reds in the 2008 First-Year Player Draft, Stewart boasts a 3-10 record and 6.82 ERA across 33 career big league appearances with the Blue Jays, White Sox and Red Sox.
Zack Meisel is a reporter for MLB.com. Follow him on Twitter @zackmeisel. This story was not subject to the approval of Major League Baseball or its clubs.