Reyes, 27, became a free agent last month when the Orioles opted not to tender a contract to the left-hander, who was arbitration-eligible for the first time in his career.
In 2011, Reyes made 20 starts for the Blue Jays before being designated for assignment in late July. He was subsequently picked up off waivers by Baltimore, which used Reyes in nine games (five as a starter) over the season's final two months.
Reyes combined to finish with a 7-11 record and 5.57 ERA.
A former second-round pick in the 2002 First-Year Player Draft, Reyes has made 70 career appearances (62 starts) in parts of five seasons in the Majors. He spent the first nine years of his professional career in the Braves organization.
Though the Pirates have already tentatively set their rotation (Charlie Morton, Jeff Karstens, James McDonald, Kevin Correia and Erik Bedard), the club has been eyeing ways to improve its pitching depth. It's questionable whether Morton (hip surgery) will be recovered by the beginning of April, and the Pirates don't have that many other Major League-ready pitchers waiting in their system.
Reyes, along with Brad Lincoln and Jeff Locke, provides the Pirates with some insurance in case Morton or anyone else is not able to be a part of that Opening Day rotation.