The club also recalled infielder Steve Lombardozzi, an Atholton grad, from Triple-A Norfolk.
Hundley, who is a career .238/.296/.389 hitter, gives the Orioles a veteran catcher with Matt Wieters on the disabled list and his status for the rest of the season uncertain.
Hundley, 30, was hitting .271 with one homer and three RBIs in 33 games with San Diego this season.
“Hundley is a proven veteran catcher,” Orioles executive vice president Dan Duquette said in a text message. “[He] handles pitchers well and has power to help stabilize [the] team.”
Patton, who opened the season serving a 25-game suspension for a positive test for an amphetamine after taking Adderall, had allowed five runs in his past two outings after six straight scoreless outings.
He was the losing pitcher in the Orioles’ 8-7 loss in 13 innings to Cleveland on Thursday and owns an 8.10 ERA.
Hundley is not expected to be added to the Orioles’ 25-man roster today, but when he is, catcher Caleb Joseph likely would be optioned to Triple-A.
A former second-round pick by San Diego, Hundley is making $4 million this season in the final year of a three-year, $9 million deal, which is why cash was involved in the trade. Hundley has a $5 million club option for 2015.
Hundley has thrown out 28 percent of base runners -- which is the league average -- over his seven-year career. He owns a .992 career fielding percentage, but led all NL catchers in errors (10) and stolen bases allowed (81) last season.
Lombardozzi, who was hitting .294/.364/.324 in 17 games at Norfolk, will officially replace Patton on the 25-man roster, so the Orioles will work one pitcher short for today’s afternoon game against Cleveland.
Right-hander Preston Guilmet is currently on the team’s 24-hour taxi squad and will be recalled when first baseman Chris Davis goes on the paternity list.
Davis’ wife, Jill, is scheduled to be induced on Sunday morning and Davis is expected to miss Sunday's and Monday’s games.