The powerful 26-year-old - who is 5ft 10in and weighs 14st 6lb - had been playing for provincial based New Zealand side Taranaki where he had played 33 games.
Hard running centre Tuifua, who last featured for Samoa in their fixture with Georgia in November last year, will link-up with his new team-mates later this week.
Director of Rugby Phil Davies said: "Isaia is a very strong ball carrier and defender who will add to our squad.
"We have brought him in initially until the end of the season to look at him and provide us with cover with the injuries we have to the back line.
"We feel Isaia's style of play will complement the likes of Dafydd Hewitt, Gavin Evans and Richard Smith and give us another option. It gives us depth in midfield as other players come back from injury."