The 24-year-old winger played one rugby league test for Tonga in 2014 but struggled to cement a career in the 13-man code and switched to union earlier this year in Australia's National Rugby Championship.
"It was a good experience as it helped me to learn more about the game from the coaches and support staff I am now working with at the ... Waratahs," Folau said of his time at the Sydney Rays.
Israel Folau also played rugby league before cementing himself as one of the best fullbacks in rugby union, having notched 71 test caps for Australia since his debut in 2013.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Australia-born John Folau acknowledged he had an eye on joining his brother on the biggest stage.
"As a kid, one of my childhood memories was sitting around the TV with family and watching the Wallabies play and watching those players represent the country," John Folau said.
"I now have the opportunity to play for a team that represents an entire state, which is humbling."
Waratahs assistant and Sydney Rays head coach Chris Whitaker said while injury restricted Folau to two games in the NRC, he had shown some natural talent and a willingness to learn the intricacies of the 15-man game.
"He is an athletic player with a strong motivation to learn more about the specifics of the game and also his playing position," the former Wallabies scrumhalf said.
"I cannot wait to see how far he can go in this sport."
The Waratahs open their 2019 Super Rugby campaign against the Wellington Hurricanes in Sydney on Feb. 16.
(Reporting by Greg Stutchbury; Editing by Nick Mulvenney)
