Leeds director of rugby Stuart Lancaster said: "We are pleased to have added to our squad, especially someone who has played for his country as recently as this summer.
"I spoke to Leinster coach Michael Cheika regarding Fosi and he could not speak highly enough of him. He adds bulk to our pack and has the ability to scrummage well, plus he carries the ball well.
"I feel he will complement the other personnel we have in our squad and we are looking forward to his arrival."
Pala'amo will arrive in the UK next week in time for Leeds' second pre-season game against Ulster in Belfast on Friday, August 31.