Giteau, capped 91 times for Australia, said he has settled terms for an 18-month contract with the Top 14 club, leaving the door ajar to return ahead of the British and Irish Lions tour to Australia in 2013.
“Signed with Toulon for 18 month deal after world cup!” Giteau said on Twitter on Tuesday.
“Love the brumbies and wanna make this yr as enjoyable as I can. Exciting times!!”
The 28-year-old will join English fly half Jonny Wilkinson, former New Zealand prop Carl Hayman and former Western Force team-mate Matt Henjak at the side coached by Phillipe Saint-Andre.
George Smith, a former team-mate of Giteau’s at the Brumbies and Wallabies, joined Toulon following the 2010 Super Rugby season but the flanker will join Japanese club Suntory at the end of this season.
Toulon are currently placed fourth in the top French division and still in the running to qualify for the 2011-12 European Cup.