The 30-year-old utility back -- capped 24 times since making his Test debut in 2013 -- has impressed for the Bears on their return to the Premiership this season scoring five tries.
"Alapati consistently shows that he is one of the most dynamic players in the competition and he will be a major asset for us going forward," Bristol Bears head coach Pat Lam told the club website.
"He is enjoying his rugby and the team environment - we're excited to see what he can accomplish in the next two years."
