"Gareth is a very significant addition to our squad and I'm delighted to confirm his arrival," said Head Coach Dean Ryan. "We have made no secret of our aim to compete at the top end of Europe, and world-class players such as Gareth will be key in making this happen.
"As soon as we learned of his availability we moved quickly to show him the Gloucester set-up. Gareth offers considerable physical presence and a dynamic carrying game, which will suit our style perfectly."
Delve made 62 appearances in six seasons at the Rec since joining from Colston's School in Bristol and is currently undergoing treatment after rupturing the anterior cruciate ligament in his left knee earlier this month against Leicester.
Ryan added: "Obviously his injury hasn't come at the ideal time, particularly for his own Rugby World Cup ambitions, but we're determined to support him fully through the rehab process and our environment is ideal to get him back playing rugby as soon as possible.
"In terms of his availability for Gloucester, we'll probably see him return to action around the same time he would have come back from the Rugby World Cup anyway, so in that respect not much has changed."
Delve made his Wales debut in the RBS Six Nations Championship of 2006, picking up two caps. He was selected for the summer tour of Argentina and was also in the 2007 RBS Six Nations squad.
Delve commented: "I've thoroughly enjoyed my time at Bath and want to thank everyone involved at the club. In particular, I would like to thank my teammates and all the Bath fans that have given me such great support over the years. Bath Rugby gave me my first full-time contract, so the decision to leave is not one that I have taken lightly.
"However, I feel that at this stage in my career I need a fresh start and a new challenge to help me realize all my ambitions in the game. After meeting Dean Ryan it's very clear the direction in which he is taking his squad and they have made significant strides in a short space of time.
"The training ground at Hartpury College is a great facility and the stadium development at Kingsholm this summer should turn the ground into an even more daunting place for away teams, so the whole package is there.
"They already have a number of quality players and I look forward to getting over this injury, improving my game in that environment, making myself a better player and taking the club even further forward. I am very excited at the prospect of facing the challenges that lie ahead of me."