The 37-year-old second row, who signed on a 12-month deal for last season, will keep going until the end of the 2007-08 campaign.
Llewellyn, who is in Spain this week with his team-mates for warm-weather training, said: "When I signed for Bristol, I honestly thought that it would be for just one year. I wanted one last crack at the Premiership and to prove that I could cut it at that level.
"But I've really enjoyed the past couple of seasons. The club has gone from strength to strength and it's been great to play a part in it. The set up here at Bristol suits me. We're well looked after and that helps me put 100% in to every game.
"Next season, we're looking to take things on a step further. I want to be part of that, I'm enjoying my rugby and feel that I can continue to contribute."
Head coach Richard Hill hailed Llewellyn, who is Wales' most capped player, as a role model for the younger Bristol players.
"Gareth is an example to the younger members of the squad.
"He is so professional in the way he looks after himself and the way he prepares for games. He's in fantastic physical shape and we had no hesitation in asking him to stay on for another year.
"Last season, he was one of our senior professionals and one of our leaders. In Matt Salter's absence, he has stepped into the breach and skippered the side. His captaincy has been inspirational and his level of play remains consistently high. He'll continue to be a valuable asset for us."