The 26-year-old two-test All Black put pen to paper on a long-term deal with the London club, completing a switch from Irish side Munster.
Harlequins director of rugby John Kingston did not specify the length of contract, but said he was delighted to get the barnstorming midfielder on a "long-term contract".
"He is a high class, quality player who is comfortable at either 12 or 13. Alongside his obvious talent, he will help balance the squad further by being available for us all season, which is just as important, given our ambitions as a club.
