Grindal, 31, whose two spells at the Tigers sandwich a seven-year stint at Newcastle, will now team up with the ambitious RFU Championship outfit.
After a difficult two years following their relegation from the top-flight, Bristol are targeting promotion - having finished top of the table at the end of the regular season.
And head coach Liam Middleton is thrilled with his new addition.
Middleton told the club's official site: "James brings with him extensive experience at Premiership level and a real energy to contribute to what we are doing at Bristol.
"He is a perfect fit, both as a player and person and I'm certain he will make a positive impact on our game and our culture."
Grindal, who has won three Premiership titles, two Heineken Cups and the Powergen Cup during his 13 years as a professional, added: "This is a club with a proud history and, having played against Bristol in the past, I know how vocal and passionate the supporters are.
"I look forward to helping the squad achieve its goals, whether we are a Championship or Premiership club next season."
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