The Nuneaton-born scrum-half is in his second spell as a Tigers player, returning after seven years at Newcastle to the club where he originally made his debut as a teenager in 1999.
Capped at England Saxons level, Grindal has made 83 appearances for Tigers and been a member of squads in four Premiership title wins including 2009/10 and the back-to-back Heineken Cup victories in 2001 and 2002, as well as winning the Powergen Cup with Newcastle in 2004.
Tigers director of rugby Richard Cockerill said: “James has been fantastic for this club in two spells. He is a very good player, I brought him back to this club in 2009 and he has shown with his performances in recent weeks that he still has a lot to offer.
“We wish him well for the future but for the moment he remains an important member of the Tigers squad.”
Grindal, 31, said: “I met [Bristol head coach] Liam Middleton and I was really impressed. It’s an exciting challenge for me at Bristol.”