The 27-year-old, who switched codes from rugby league to union to join Toulon in the French Top 14 in 2017, currently plays for Bordeaux and scored two tries for Fiji at the recent World Cup in Japan.
"There is no doubt that Semi is one of the best players in the world â€“ he's truly a world class performer and we are so excited about what he can do for the team," Bristol director of rugby Pat Lam said in a statement on the club website https://www.bristolbearsrugby.com/news/world-class-radradra-pens-bears-deal.
"Every top club across both codes on the planet wanted him, so for Bristol to be able to bring in a player of his quality underlines the fantastic work that the club is doing.
"As a person and a player, he is perfect for our culture which places an emphasis on community and high performance."
Radradra began his professional career with Parramatta Eels in Australia's National Rugby League (NRL).
"I'm looking forward to coming to the Premiership and to be playing under Pat Lam at Bristol Bears," he said. "They are one of the most exciting teams in Europe right now and the way that they are doing things is making everybody take notice."
Bristol currently top the Premiership standings with 14 points after winning three and losing one of their four matches so far.
(Reporting by Simon Jennings in Bengaluru; Editing by Alex Richardson)