The 26-year old has endured a torrid injury run throughout his rugby league career, playing in 73 NRL games since making his debut in 2012.
Moga also brings international experience having played three Tests for Samoa.
Rabbitohs Head of Football Mark Ellison said Moga would be a valuable addition to the club.
“A couple of injuries have stalled him throughout his career but when he has been fit to play he has proven that he can be one of the most destructive outside backs in the game,” he told the club website.
“He has been part of winning squads throughout his NRL career and has played Test football.
“We’re looking forward to having him join our squad this pre-season and seeing him push for first grade selection on a weekly basis.”