The cyclist partnered with the Welsh brewer to launch the ale just as this year's Tour de France kicked off last Saturday.
Geraint was closely involved in the development of the beer, called "G", creating a 4 per cent ABV ale that is said to be, "light, refreshing with crisp citrus flavours."
The beer has an especially light golden colour inspired by the famous yellow jersey and features a picture of Geraint in action in last year's Tour.
"I couldn't quite believe it when Brains approached me with the concept of brewing the beer," Geraint said.
"I'm Cardiff born & bred, and Brains is part of both Cardiff's and Wales' identify, so I was incredibly proud when they asked me to get involved. It's been a real eye opener, and a lot of fun getting involved with the development of the beer, and of course the tasting...it's a tough job but somebody has to do it!
"I'm delighted with the result and am already looking forward to sinking a pint or two at my local Brains pub once the Tour is over!"
Geraint, who won the Tour last year, has also been voted BBC Sports Personality of the Year, won two Olympic gold medals and a Commonwealth gold.
Brains will be screening the race in a number of its pubs and is offering customers the chance to win a yellow jersey signed by Geraint, as well as other "G" related prizes.