The Australian-born Balfour was 0-2 with a 20.25 ERA in three relief appearances with the Brewers this season.
"Grant Balfour is a guy that we've liked for some time," said Rays executive vice president of baseball operations Andrew Friedman. "He's had some injury problems. But we feel like he's healthy and he's capable of helping us out at the Major League level. He's a guy who has good command, strikes out a lot of guys and competes extremely well, so we're excited to add him to the mix in the bullpen.
"Obvioulsy, we've had a lot of struggles out there this season, and we're going to try and be aggressive to shore it up the best we can for the last two months of the season and going into the offseason."
The Brewers were forced to part with Balfour, who was out of Minor League options, because they needed roster space Friday for reliever Scott Linebrink, whom they acquired in a trade with San Diego on Wednesday. Linebrink had a family engagement that kept him from joining the Brewers until Friday afternoon.
Balfour underwent two major surgeries in 2005 but worked his way back to the Majors Leagues. He appeared in 24 games this season for Triple-A Nashville, where he was 1-1 with five saves and a 1.69 ERA. He also pitched in eight games for Double-A Huntsville and was 0-0 with two saves and a 2.38 ERA. All told, he has struck out 91 batters in 46 innings in 2007.
"It didn't really go as planned," Balfour said.
Because Balfour had no remaining options, the Brewers were limited in what they could do.
"He was a guy I was thinking about counting on for next year," said Brewers general manager Doug Melvin. "You look at other teams' bullpens, and that's the kind of arm that teams get and, all of a sudden, they find it. It clicks."
McClung, 26, pitched in 85 games for the Rays from 2003-06 and went 17-24 with a 6.28 ERA. He has a 1.99 ERA in 40 appearances this season for Triple-A Durham.
"Seth's been in the organization for a long time and we wish him nothing but the best of luck, and he's got the type of arm where a second organization can get lucky with him where he can go on to have a lot of success," Friedman said. "And we're rooting for him."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.