The 26-year-old number eight will link up with the Aviva Premiership club from New Zealand provincial side North Harbour.
"Viliami is a really good ball carrier, and we are delighted that he has agreed to join us," Sale rugby director Steve Diamond said.
"He could have delayed coming here and gone with Tonga on their current autumn tour of Europe, but instead he has joined up with us straightaway which is a great boost for us.
"We had been looking for another back-row player all summer and we identified Viliami playing over in New Zealand for North Harbour in the ITM Cup. He comes highly-recommended from our contacts over there."
- Sports & Recreation
- Aviva Premiership