Gallagher (0-0) made 15 relief appearances for the Padres this season before being designated for assignment on July 1. He boasted a 5.40 ERA with 21 strikeouts and 19 walks.
"I'm very grateful that the Pirates were able to give me the opportunity to come in there and really get a chance to show what I can do," Gallagher said.
The 24-year old struggled with control issues at times this season, but Gallagher said he knows it can be fixed.
"I think it's more just being able to get some consistent work and being able to get back to the basics of just throwing," Gallagher said. "I've been a little erratic at times this year. I think I can definitely get that problem solved and be a big help."
Gallagher is 8-9 with a 5.57 ERA in 60 career games. He played two seasons apiece with both the Cubs and Athletics before going to the Padres in 2009.
San Diego manager Bud Black said he sees potential in Gallagher.
"I think we all figured that Sean would end up with a Major League club," Black said. "He's a very talented pitcher. I'm happy for Sean. I hope he gets an opportunity to pitch and I hope he pitches well."