English Premiership side Bristol said Thursday that Rees-Zammit was joining with "immediate effect" ahead of the new league season.
The 24-year-old has scored 14 tries in 32 Tests for Wales and was selected for the 2021 British and Irish Lions tour of South Africa.
Rees-Zammit, formerly with rivals Gloucester, said in a Bristol statement: "Bristol Bears was the obvious choice for me.
"The style of rugby the team play - all about entertaining the fans and growing the game - perfectly aligns to how I want to play and how I express myself on and off the field."