Gausman, 28, is 3-7 with a 6.19 ERA in 16 starts this season. He went 5-3 with a 2.87 ERA in 10 starts after the Braves acquired him from Baltimore last year.
The Reds will take on the rest of Gausman’s $9.35 million salary for 2019. He is arbitration-eligible this offseason and the Reds can either keep him or, by non-tendering him, make him a free agent.
Gausman will wear No. 46 with the Orioles.
Gausman is 47-61 with a 4.30 ERA in 176 games (153 starts) since debuting with the Orioles in 2013.
Cincinnati is 52-58 this season. The Reds are 7 1/2 games behind the first-place Chicago Cubs in the NL Central.
Atlanta is 66-47 this season. The first-place Braves have a seven-game lead over the Phillies and Nationals in the NL East Division.