The ambitious French Top 14 side unveiled the 31-year-old former international 24 hours after he was released by Hull KR just six months into a three-year contract.
In a statement on their website, the club said: "The international treiziste Willie Mason committed with the Toulon Rugby Club for a season, with an option for a second year.
"After a season in Britain, Willie Mason has decided to start a career in rugby union and chose the Toulon Rugby Club.
"He will also participate in the Barbarians' summer tour in England and Wales."
Today's announcement brings to an end the long-running saga of Mason's brief but colourful spell in Super League.
The former Canterbury, Sydney Roosters and North Queensland forward signed a two-year deal with Rovers with an additional 12-month option, but made just six appearances for them before they lost patience with the wayward star.
The Robins stopped paying his wages, citing a breach of contract after discovering the player had travelled to France to have talks with Toulon owner Mourad Boudjellal without their knowledge or permission.
The Auckland-born Mason, who won 24 caps with the Kangaroos, is following in the footsteps of another controversial former NRL star, Sonny Bill Williams, who reneged on his contract with Canterbury Bulldogs to sign for Toulon.
The big-spending French club can already boast England fly-half Jonny Wilkinson and Wales centre Gavin Henson in their star-laden squad and, after a disappointing season in which they failed to reach the play-offs, they have also recruited Wallaby back Matt Giteau, Springbok World-Cup winning lock Bakkies Botha and French winger Alexis Palisson for next year.
Although he has never played rugby union, Mason has revealed he is set to play for the Barbarians against England at Twickenham next Sunday and against Wales at the Millennium Stadium on Saturday, June 4.
"I'm just so excited to be playing rugby and I'm looking forward to the challenge," he told the Sydney Morning Herald.
"I've certainly jumped in the deep end in agreeing to play for the Barbarians without having any rugby experience.
"I've just got to learn how to play rugby in a hurry and how to speak French because I don't know a word yet and they say it's not that easy to pick up."
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