"They had a catching need and we had a catching surplus," Byrnes said.
Patton, 28, will be in uniform Sunday for the series finale against the Cubs. He comes to the Padres as a second left-hander in the bullpen with Alex Torres. He'll need to pitch well to stay. Patton doesn't have Minor League options.
"Here's a guy that you look at the little bit of track record that he's had over the past three years, overall it's been pretty good work," Padres manager Bud Black said. "He's done a nice job against left-handed hitters and he's capable of getting the right-handed hitters out. He complements Torres.
"He's a guy that depending on his performance can play a role in a very good bullpen."
Patton has a career 3.28 ERA in 143 big league games over six seasons with the Astros (2007) and Orioles (2010-14). He has an 8.10 ERA in nine appearances this season.
In December, Patton was suspended for the first 25 games of the season after his second positive test for amphetamines. He was reinstated from his suspension May 1.
This wasn't the first time Patton landed on the Padres' radar, Byrnes said.
"We actually talked a little bit about him during the offseason, when we were rebuilding our left-handed depth. It turned out this is where it matched up. Patton, I think, will be a good fit for us as a second lefty back there," Byrnes said. "With the current state of affairs with the injuries, we wanted an experienced arm."
Patton's best season with the Orioles came in 2012 when he posted a 2.43 ERA in 55 2/3 innings over 54 games. He had a 3.70 ERA in 56 games a year ago.
The Orioles needed a catcher after Matt Wieters landed on the disabled list on May 11 with a strain of the right elbow.
"This one sort of makes a lot of sense. Baltimore's right in the middle of a race, and their normal catcher got hurt. He'll [Hundley] be able to contribute more for them than he was doing for us. And obviously Patton can help us."
The Padres reportedly sent less than $2 million of the $4 million Hundley is owed this season. The Orioles will have a $5 million club option on Hundley to decide on after the regular season.
Patton is making $1.1 million this season and is still under team control for two more seasons.
Patton joins a bullpen which has the second-lowest ERA (2.17) in the Major Leagues going into play on Saturday, trailing only the Nationals (2.13).