Fihaki, who can also play on the blindside flank, currently has three Tongan caps to his name having made his Test debut against Japan in May. He was due to tour Europe with his country and could have been involved in their games against Romania, France and Wales but instead opted to join up with the Sharks with immediate effect.
"We believe that Viliami will be a really good addition to our ranks and that he will add something extra to the squad," said Sale boss Steve Diamond.
"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.
"Vilaimi's a really good ball carrier and we're delighted that he has agreed to join us. He could have delayed coming here and gone with Tonga on their current Autumn tour of Europe but instead he's has joined up with us straight away, which is a great boost for us.
"He's signed with us for three years and we will be looking to get him involved straight away here."
Sale's LV= Cup tie at Harlequins tomorrow night is likely to come too soon for Fihaki but he could feature when the Sharks return to Aviva Premiership action at home to Worcester a week on Friday. Diamond's men then travel to Saracens on November 30 before getting their Amlin Challenge Cup campaign back underway against French Top 14 team Oyonnax next month.
Sale have started their quest for a third Amlin Challenge Cup crown with a bang, beating 2012 tournament winners Biarritz Olympique at the AJ Bell Stadium before taking maximum points against Worcester Warriors at Sixways.
Those results put them in the box seat in Pool 1, leaving them with a four-point advantage over Biarritz ahead of the December double headers.