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Smith debuted for the Brumbies in 2000 and notched 128 caps before heading overseas in 2010 after retiring from international rugby. He returns to Canberra after stints with French clubs Toulon in 2010 and Stade Francais last year.
The 32-year-old, one of Australia's finest loose forwards in a glittering 110-cap career, will start training with the Canberra-based side next week and could be considered for selection for their home game against the New South Wales Waratahs on March 9.
"It's exciting news for the club that we have been able to bring George back to Canberra," Brumbies coach Jake White said in a statement on the team's website on Monday.
"He's achieved a great deal in the game and was an important part of the team for many years, so naturally everyone at the Brumbies will be pleased to have him back.
"George's record and ability to win games is outstanding. The guy has won Super Rugby championships, he's won big games for the Wallabies and he's played in a rugby World Cup final so I want him to bring that winning culture back here to the Brumbies."
Smith, who played his first Australia test in 2001 against France, bolsters an already formidable back row at the Brumbies alongside fellow Wallabies flanker and new recruit David Pocock.
"Everyone wants to play alongside George Smith ... He's playing some very good rugby," Pocock told reporters in Sydney on Monday.
"You can always learn off George. Apart from the on-field stuff, off the field he's a great guy to have around as well."
The Brumbies are top of the table in the 15-team southern hemisphere provincial championship after winning their first two matches.