The Otago Highlander played 10 of his 24 tests last season but was overlooked for most of the British and Irish Lions series before playing a late cameo in the drawn third test.
"I wanted to let you all know that after a lot of thought I have decided to join Toulon (RCT) at the end of the year," the 25-year-old said on his Instagram account on Friday.
"It has been the hardest decision of my life to leave New Zealand, but after looking at all the things that are important to me I feel this is the right time for me personally and for my career."
The powerful midfielder will join former All Blacks Ma'a Nonu and Luke McAlister at the French glamour club but declined to elaborate on the length or terms of the deal.
"I hope to return one day with more experience and success," he said.
Fekitoa will start in the Highlanders' Super Rugby quarter-final against the Canterbury Crusaders in Christchurch on Saturday and was a driving force in the Otago side's run to their maiden championship in 2015.
His departure will almost certainly curtail his international career, with New Zealand Rugby ruling overseas-based players ineligible for selection except in rare cases.
All Blacks coach Steve Hansen told local media on Thursday he had hoped Fekitoa would stay but would support his decision either way.
"If they're in two minds, you try and steer them to making a decision that's right for what they're thinking versus the money. Sometimes it's a hard battle," Hansen said.
(Reporting by Ian Ransom in Melbourne; Editing by Andrew Both and Larry Fine)
