The Pirates designated Stewart for assignment last week to make room on their 40-man roster for Jeff Karstens.
This will mark Stewart's second stint with the White Sox. Chicago acquired the right-hander from the Toronto Blue Jays along with Jason Frasor in exchange for Edwin Jackson and Mark Teahen in 2011. They then shipped him to the Boston Red Sox with Brent Lillibridge in the Kevin Youkilis trade.
Stewart spent the 2012 season with the White Sox and Red Sox, maintaining an ERA of 8.58 over 20 appearances, including three starts. The 26-year-old also made 11 starts for the Red Sox Triple-A affiliate, posting a 3.94 ERA.
Over parts of two seasons at the major league level, Stewart owns a 6.82 ERA with 5.6 strikeouts and 1.9 walks per nine innings. He's posted an ERA of 3.19 over five seasons in the minors.