Bristolian Tavare 26, has signed until the end of the 2019 season, while South African Van Buuren, also 26, has signed until the end of the 2018 campaign.
"It's great to see both players committed to the club for the long term," said head coach Richard Dawson.
"Will and Graeme are still developing their skills and will add strength and depth to the squad."
Van Buuren was signed from South African side Titans earlier this year and qualifies as a non-overseas player because his wife has a British passport.
"I have had an awesome time at the club this year and loved every minute of it on and off the field," he said.
"I am over the moon and very grateful to be spending the next couple of years in Bristol and can't wait for the 2017 season to start."
Batsman Will Tavare and all-rounder Graeme van Buuren have extended their contracts with Gloucestershire.