The 30-year-old Hundley hit .271 with a home run and three RBI in 33 games with the Padres this season. The Orioles are currently operating without starting catcher Matt Wieters, who's on the disabled list with a strained right elbow.
In seven seasons with San Diego, Hundley batted .238 with 47 homers and 195 RBIs in 510 games. He also has a .990 career fielding percentage and a 28 percent success rate in throwing out would-be base stealers.
"Hundley is a proven veteran catcher who handles pitchers well and has power," Orioles vice president of baseball operations Dan Duquette said in a text message to The Associated Press.
Duquette said the trade helps "stabilize the team."
Baltimore had been using Steve Clevenger and Caleb Joseph behind the plate in Wieters' absence. Clevenger entered Saturday's game against Cleveland with a .246 batting average, no homers and eight RBIs. Joseph was 1 for 20 in seven games.
Patton was 0-1 with an 8.10 ERA in nine appearances with Baltimore. He missed April while serving a 25-game suspension for testing positive for a banned amphetamine last year.
Patton was one of four left-handers in the Baltimore bullpen. Opponents were batting .333 against him this year.
The Orioles also received cash considerations from San Diego in the deal.
With Patton off the roster and Hundley not yet with the team, Baltimore recalled infielder Steve Lombardozzi from Triple-A Norfolk before Saturday's game. Acquired in a March 24 trade with Detroit, Lombardozzi hit .292 in 19 games with the Orioles before being optioned to Norfolk on May 1.