The 22-year old has played nine NRL games after making his debut with the Tricolours in 2018 before joining Red Hill the following season.
He made just three first-grade appearances for the Broncos in 2020 during their wooden-spoon campaign.
Warriors head coach Nathan Brown was excited to add the half to his mix for 2021.
“It’s great to be able to bring in a player of Sean’s ability to boost our squad,” said new Brown told the club website.
“He’s been part of the great system at the Roosters and has broadened his experience spending the last two seasons with the Broncos. He’ll add real value for us and ensure there’s real competition for places.”
O’Sullivan becomes the 29th player for the Warriors for next season meaning they have one more roster spot to fill.
He is the son of Warriors recruitment and development manager Peter O’Sullivan