The Samoan international lock, 31, has penned a one-year deal with the Aviva Premiership club.
Leo began his career with London Wasps before moving to France in 2010.
"I'm excited to be moving back to England to join London Irish," he told the club's official website.
"I always enjoyed playing in the Premiership and I'm excited to go back to compete in that competition again. Irish are building nicely for the future it I'm looking forward to being part of it."
Irish return to action on Sunday when they entertain Newcastle Falcons in the Aviva Premiership.