Reyes, 26, went 5-8 with a 5.40 ERA in 20 starts for the Blue Jays this season. He spent parts of the previous four seasons with the Atlanta Braves.
In 61 career major league games (57 starts), Reyes is 10-23 with a 6.04 ERA in 304 innings.
To make room for Reyes on the 40-man roster, the Orioles have transferred Luke Scott to the 60-day disabled list.
Reyes will be added to the 25-man roster, but not in time for tonight's game.
Also, Brad Bergesen came off paternity leave today, and the Orioles finally released Justin Duchscherer.
Duchscherer signed with the Orioles as a free agent over the winter, but didn't appear in a game. The last we heard, he was undergoing another hip surgery.
Update: Reyes will be used out of the bullpen initially, but he could join the rotation later.
No decision has been made regarding a corresponding roster move tomorrow. Left-hander Troy Patton could be optioned, but that's only one possibility.