The 31 year old, who played the first of his 37 tests in 2013, is out of contract after being released by the Bristol Bears in May.
Lam, who had been at the club since 2014, captained the side in the early part of the 2016-17 English Premiership campaign but was one of 11 players to be shown the door by his coach and cousin Pat Lam, including fellow Samoa international Tusi Pisi.
He received offers from a couple of clubs in France but said he turned them down in order to play for Samoa.
"It was a no-brainer for me.
