The 26-year-old who has played last season for Carmarthen Quins in the Premiership has been in pre-season training with the Scarlets over the fortnight following on from his eye-catching performances for Tonga in the Churchill Cup including a try over England Saxons in June.
With three international Test caps, Timani, who has played at clubs in New Zealand and Japan in his career, is highly regarded by the current Tongan coaching team. He's expected to further his international experience in the World Cup this summer where Tonga open their campaign against the All Blacks and he will be taking part in the Pacific Cup in coming weeks.
Sione is the brother of Waratahs lock Sitaleki Timani.
At near 19 stone (120 kg) and 6' 5", securing the heavyweight forward will bring a physical presence to complement the current Scarlets pack.
"Sione brings physicality and a real presence to our second row options and bulk which you need in the pack to complement what we have in our group already," said Scarlets head coach Nigel Davies.
"He's a great acquisition for us in terms of the balance of our pack, he's got a lot to bring, adding a different dimensions to our forwards - with plenty scope to develop more from being within our set-up. We're confident that he will strengthen further as player once within our system.
"He's also shown he's very committed and loyal player and puts in the graft on the pitch and that he's looking to score tries as well. We are delighted he has joined us and look forward to his contribution to the group next season."
Living in Swansea with his wife and two young daughters aged two and three and known to be a humble and gentle giant off the pitch, Timani has been one of the stand out players of the Premiership last season and says he's looking forward to joining the Scarlets.
"This is a great opportunity for me to play for such a respected and well-known club and I hope to fit in well with the group and the players," he said.
"The Scarlets are a fresh side full of ambition and quite obviously have a philosophy to give the younger developing players a chance, which appeals a lot to me. They play some great rugby and I'm looking forward to being part of the team."
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