Joe Marchant will join the Auckland Blues on a six-month deal next year, as part of a sabbatical clause written into his Harlequins contract.
The 23-year-old will link up with the Super Rugby side between January and June 2020 before returning to the Stoop.
He said: “Firstly, I could not be more grateful to Harlequins for allowing me this opportunity to expand and grow my game.
“The club has supported me every step of the way in my career to date and proven that Harlequins is still willing to support my growth as an individual.
“I am really excited to be given the chance to experience playing in Super Rugby. I am sure that I will learn a lot and I can’t wait to bring my learnings back to the club.”