A strong-carrying, athletic forward, Lousi stands at an imposing two metres and weighs in at more than 19 stone.
He is the younger brother of Tonga rugby league international Sione Lousi.
“I am very excited to join the Scarlets for next season,” he said.
“It is a team that plays an exciting brand of rugby and I am looking forward to linking up with a group of players that are ambitious to be winning trophies.
“Hopefully, I can help them push forward and achieve further success over the next few years.”
The Scarlets have already announced that Wales Grand Slam heroes Jonathan Davies, Ken Owens and Rob Evans have signed new contracts, along with full-back Johnny McNicholl.
The signing of a player of Lousi’s calibre is another statement of the region’s intent for next season.
“Sam is a great addition to the squad,” said Scarlets general manager of rugby Jon Daniels.
“He is a strong, aggressive ball carrier and also possesses the handling skills that fits in with our style of rugby at the Scarlets.
“He has played in a lot of high-profile matches for the Hurricanes and was an ever-present last season.
“We look forward to welcoming him on board.”