The Orioles made room for Pearce on the 25- and 40-man rosters by designating Bill Hall for assignment.
Pearce, 29, was batting .321 with 15 doubles, 11 home runs, 30 RBIs and a .996 OPS in 52 games for Triple-A Scranton/Wilkes-Barre. He opted out of his contract, and the Orioles sent cash to the Yankees to cover the deal.
Pearce batted .232 with 29 doubles, three triples, nine home runs and 52 RBIs in 185 major league games over five seasons (2007-2011) with the Pirates.
Hall batted .222 (2-for-9) with one home run, one RBI and two runs scored in seven games with the Orioles. He started in right field last night.
The Orioles have outrighted Hall twice this season. They purchased his contract again yesterday.
Hall can refuse another outright assignment if he clears waivers.
"We designated Bill Hall for assignment. We'll find out about him Monday at 1 o'clock. And Steve Pearce will join us today," said manager Buck Showalter.
"He's on our 25- and 40-man rosters, obviously. He was playing in Scranton and had an out in his contract and exercised it. He'll be here shortly.
"I know him a little bit from spring training. Our people thought he has the potential to help us, outfield and first base. He was second or third in the International League in hitting. Trying to continue to do some things to offset some of the injuries, and hopefully he'll help."
Pearce represented a better option than the players at Triple-A Norfolk, especially with the Orioles preferring to keep Xavier Avery in the minors so he can continue to develop.
"We've got some good options there," Showalter said. "Lew Ford's still trying to really establish himself from the Atlantic League to that level. He's had some injury stuff here and there. From what I can understand gathering all the information and knowledge, that they thought (Pearce) was our best option, and we thought that.
"He was playing against Norfolk last night. They got rained out. Every team in our organization got rained out except us."
As for the preference to keep Avery in Norfolk's everyday lineup, Showalter said, "That's always been the case, but also when he was here, we didn't have Endy Chavez. I think that's it more than anything. Endy's going to get it going, and he's going to help us here. I really believe that."