To land Wolf, the Astros sent minor league right-handed pitcher Chad Reineke to the Padres. By making the trade, the Padres saved $3.7 million.
Wolf, 31, is 6-10 with a 4.74 ERA this season. He began his career with the Philadelphia Phillies -- the former employer of Ed Wade, the current general manager for the Astros.
The left-handed Wolf will start for the Astros in Milwaukee on Sunday, Wade said.
"I've known Randy since the day he signed his first professional contract," Wade said. "I can say without hesitation that he brings every quality that you'd want in a member of your team, both on and off the field."
Wolf, in Cincinnati where the Padres were playing the Reds, was caught off-guard by the trade.
"When it ultimately happens, you're shocked a little bit," Wolf said. "I didn't really think about it. I think any team would have been, in some way, a surprise."
Earlier this week, the Astros were forced to place veteran pitcher Roy Oswalt on the disabled list with hip and back trouble.
Padres general manager Kevin Towers said the Astros were intent on acquiring Wolf.
"Houston was pretty aggressive, probably more aggressive than any other club," Towers said.
Wade told Wolf that he still thinks Houston can turn things around.
"He still believes in the Astros," Wolf said. "I think he still thinks there's a shot. With a little more than two months left in the season, there's an opportunity there he's trying to take advantage of. He's not giving up hope. I'm up for the challenge."
Wade was the Phillies' general manager from 1998-2005. He tried to sign Wolf for the Astros in December, but Wade said Wolf wanted to play closer to his home in California.
Wolf had Tommy John surgery in 2005 and missed the second half of the 2007 season with pain in his throwing shoulder. His 21 starts this season were tied for the major league lead and Wade has been assured by athletic trainers that Wolf is healthy.
"The feedback we got, trainer-to-trainer discussions that took place today, confirmed that he's 100 percent," Wade said.
Wolf has lost six of his last seven starts. He gave up four runs on eight hits in Saturday's 6-5 loss in St. Louis, his third straight road loss.
"In the time I had here, obviously we didn't win a lot of games," Wolf said. "Part of that is definitely my fault. I wasn't as consistent as I wanted to be. But I did have a great time here."
The deal gave the Padres the opportunity to get a prospect and save some money. Wolf signed a one-year contract last December that includes a $4.75 million base salary and a chance to make up to $9 million overall if he reaches all of his incentives.
"I'm not going to lie," Towers said. "Certainly from a financial standpoint, the club is going to save some money."
Reineke is 5-9 with a 4.41 ERA in 19 starts for Triple-A Round Rock this season. He was originally selected by the Astros in the 13th round of the 2004 draft and he is 28-32 with a 3.72 ERA in five minor league seasons in the Houston system. He has yet to pitch in a major league game.
Wolf has a no-trade clause that includes 14 teams to which he cannot be dealt without his permission, but the Astros were not among those.
Wolf will become a free agent after this season. Wade said Wolf could earn up to $3.5 million this season with performance bonuses and the Astros may look to re-sign him for 2009.
"If nothing else, this gives us the short-term help that we need," Wade said. "It also gives us an opportunity to get our foot in the door with Randy and if he pitches well enough that we want to extend the contract, all that's done against the backdrop of what's going to be a pretty thin pitching market out there."
Wolf said he'd also be interested in returning to San Diego next season, if the Padres would take another chance on him.
"I'm not leaving here with any sour taste in my mouth," he said. "I have a lot of good memories here."
Information from ESPN The Magazine's Buster Olney and The Associated Press contributed to this report.