In return, the Diamondbacks received minor league outfielder Jason Heinrich.
Bourn becomes the second outfielder the Orioles added today, with the first being Drew Stubbs, who came over via a waiver claim.
It makes sense that Baltimore wanted to beef up their outfield, considering that they rank 24th in the Major Leagues in fWAR from that position, mostly due to poor defense numbers.
Currently, the Orioles have Adam Jones, Hyun Soo Kim, Mark Trumbo and Nolan Reimold as their main outfielders on the 25-man roster.
Bourn, though, will be a defensive upgrade that is also a threat to steal bases.
This season, the 33-year-old is hitting .261/.307/.362 with three home runs and 30 runs batted in over 358 plate appearances, adding 13 stolen bases. Per FanGraphs, he has been worth -0.1 wins above replacement over his 89 games played.
Bourn is finishing off a four-year, $48 million deal that he signed with the Indians prior to the 2013 season. He was then traded to the Braves in 2015, who released him earlier this year.
Bourn then signed with the Diamondbacks on a minor league contract midway through this season. He is making the big league minimum from Arizona, and he will continue to do so from Baltimore once the trade is finalized. Regardless, Bourn will be a free agent at the end of the season.
Heinrich, 20, has hit .231/.301/.347 with three home runs and 13 RBI in 163 plate appearances at Class-A Short Season Aberdeen. He was the Orioles' 5th round pick in last year's draft.