The 34-year-old, who has played 103 tests for Australia, enjoyed a hugely successful and lucrative stay in France, playing a major role as the wealthy southern club won the European Rugby Champions Cup three times.
Suntory were once coached by England coach Eddie Jones, who handed Giteau his test debut in 2002 and previously employed George Gregan, while Wallabies flanker George Smith is still on the books at the club.
"I am very much looking forward to becoming a member of Suntory Sungoliath next season," he said in a news release.
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"Friends including George Smith and George Gregan, when they talk about the team, do so with great affection.
"Me and my family feel that it is time for a new challenge in Japan, and I believe this club is a good place to do that."
Giteau joined Toulon in 2011 after falling out of favour with Wallabies coach Robbie Deans but saw his international career revived when a change was made to allow players based overseas with 60 caps or more to represent Australia.
The rule, pushed through by current Wallabies coach Michael Cheika, was dubbed "Giteau's Law" and allowed the flyhalf-inside centre to play a leading role in Australia's run to the final of the 2015 World Cup.
He returned to play for Australia in the Rugby Championship last season but lasted only 11 minutes of the first match against the All Blacks in Sydney before hobbling off with an ankle injury.
One of the highest paid players in the world, Giteau was player-coach in the final season at Toulon with responsibility for the team's backline.
The Top League season begins in August.
(Reporting by Nick Mulvenney; Editing by Ian Ransom)
