The 29-year-old lock or flanker has 11 caps and is moving to the Premiership from the New Zealand Rugby Union.
Tonga-born Fifita, who is 6ft 5ins (196cm) tall, has won the Super Rugby title with Hurricanes and has also played for the Wellington Lions.
"I am proud to be joining one of the most historic rugby clubs in the world on what promises to be a very exciting journey," Fifita said.
Head coach Lee Blackett said signing an experienced international like Fifita was "exciting".
