The Orioles announced the move before Sunday's game against the New York Yankees. They also recalled righty Oliver Drake from Triple-A Norfolk and designated lefty T.J. McFarland and outfielder Julio Borbon for assignment.
Baltimore entered Sunday with a 70-59 record and sole possession of the second wild-card spot in the American League.
Hunter was 2-2 with a 3.74 ERA in 21 games for Cleveland this season. He was in the minors rehabbing a recent back injury when the Indians cut him on Thursday.
The 30-year-old Hunter played for Baltimore from 2011-15, going 21-20 with a 4.22 ERA. He said he was thrilled to rejoin the Orioles, adding there were "a lot of smiles, a lot of hugs" when he walked into the clubhouse.
Manager Buck Showalter said Hunter brings a lot of experience, having spent so much time in the AL East.
"Felt fortunate to add him at this time of year," Showalter said.
Drake has pitched four games for Baltimore this year, giving up six earned runs in 5 2/3 innings.
McFarland was 2-2 with a 6.93 ERA in 16 games. Borbon went 4 for 13 in six games.