Dombrowski said the Red Sox are getting Pearce and cash considerations from Toronto in exchange for minor league infielder Santiago Espinal.
Pearce, 35, has hit .291 with four home runs and 16 RBIs in 26 games for Toronto this season. he has a career average of .255 and 83 career home runs and 268 RBIs.
Boston manager Alex Cora said the Red Sox are adding a strong batter they can use against left-handed pitching.
“It's one more weapon that we have. We got better tonight,” Cora said. “It's good to have him.”
Pearce was a big part of the Orioles’ AL East-winning season in 2014, when he batted .293 with 21 home runs and 49 RBIs in a career-high 102 games.
The Orioles purchased Pearce from the Yankees in June 2012, then lost him on waivers to the Astros the next month before reclaiming him on waivers from the Yankees that September.
Pearce became a free agent in November 2015 and signed with the Tampa Bay Rays two months later before being traded back to the Orioles in August 2016 for Jonah Heim. He was granted free agency after that season.
He totaled five seasons for the Orioles, batting 255/.337/.473 with 46 home runs and 122 RBIs in 882 at-bats.
The Red Sox will be his seventh major league team, including his fifth in the AL East East.