The Orioles also recalled infielder Steve Lombardozzi, who takes Patton's spot on the 25-man roster. The Orioles have 11 pitchers today.
Hundley will be added later, perhaps Monday in Milwaukee.
Hundley, 30, batted .271 with a home run and three RBIs in 33 games for the Padres. He's a career .238 hitter in seven major league seasons, with 47 home runs and 195 RBIs in 510 games.
In 467 games behind the plate, Hundley has registered a .990 fielding percentage and thrown out 130 of 470 runners (28 percent) attempting to steal.
"Hundley is a proven veteran catcher," executive vice president Dan Duquette wrote in a text message. "He handles pitchers well and has power to help stabilize the team."
The trade provides insurance for the Orioles if Matt Wieters undergoes ligament-reconstructive surgery on his right elbow. Wieters remains on the disabled list and hasn't been cleared to throw. He's still hoping to avoid the procedure and return later this summer.
The Orioles could option backup Caleb Joseph to make room for Hundley, a right-handed hitter and former second-round pick in the 2005 First-Year Player Draft.
Hundley is making $4 million this season. His contract includes a $5 million team option for 2015.
Patton, 28, was 0-1 with an 8.10 ERA in 6 2/3 innings over nine games. He missed the first 25 games while serving his suspension for amphetamine use.
Patton allowed five runs in 1 2/3 innings in his final two appearances. He's out of minor league options and couldn't be sent down without clearing waivers.
The Orioles acquired Patton and four other players from the Astros on Dec. 12, 2007 for shortstop Miguel Tejada.
Lombardozzi, 25, is batting .292 with two RBIs in 19 games with the Orioles. He hit .294 in 17 games with Norfolk.
Update: We're still scoreless after four innings.
Ubaldo Jimenez threw 52 pitches in the first two innings and only 24 in the next two. He struck out three of the first five batters, but he also walked three of the first eight.
Adam Jones singled in the first inning and has reached base in 18 consecutive games.