Reynolds has a pulled muscle in his rib cage. He is batting .191 with two homers and nine RBIs.
The injury occurred during batting practice Friday, manager Buck Showalter said.
Reynolds said Saturday, "It's not too bad, but it's one of those things where if I push it it'll get worse."
Reynolds is the third player placed on the DL by the Orioles in a three-day span. Outfielder Endy Chavez (left oblique) was shelved on Thursday and reliever Matt Lindstrom (middle finger) was added to the list Friday.
The Orioles purchased the contract of utility player Bill Hall from Triple-A Norfolk on Saturday. He started in left field against Tampa Bay.
To make room for Hall on the 40-man roster, Baltimore designated right-hander Brad Bergesen for assignment.