Langi was released by Australian side Penrith Panthers earlier this month, with Leigh giving him a short-term deal with the option of a further year.
The 24-year-old, who has seven caps for Tonga, is versatile and can play in the back-row, at halfback or at centre.
"He is a true footballer who covers a number of positions but plays them very well," said head coach Neil Jukes.
"He is a big, strong, athletic player with a hunger to succeed and he will add a wealth of size, speed and strength to our back-line or middle unit."
Langi could make his debut for bottom-of-the-table Leigh when they play Warrington in the Super League on 7 July.
Leigh have signed Tonga international Samisoni Langi on a contract until the end of the season.