Jacinda Ardern has been appointed a trustee of the Prince of Wales' prestigious environment award, Kensington Palace has confirmed.
Prince William said it was an honour to welcome the former leader of New Zealand to the Earthshot Prize.
Ms Ardern resigned in January saying she no longer had "enough in the tank" to lead the country.
The former prime minister said she was "humbled and excited to be working with the Earthshot team".
Speaking of the appointment, Prince William said Ms Ardern's "life-long commitment to supporting sustainable and environmental solutions" and her experience would "bring a rich infusion of new thinking to our mission".
"Four years ago, before the Earthshot Prize even had a name, Jacinda was one of the first people I spoke to, and her encouragement and advice was crucial to the prize's early success.
"I am hugely grateful to her for joining us as she takes the next steps in her career," he added.
The Earthshot Prize was created by Prince William to fund projects that aim to save the planet.
Last year's awards in December, hosted by Prince William and Princess Catherine, saw five prizes handed out to winners from the UK, India, Australia, Kenya and Oman.
On joining Earthshot's board of trustees, Ms Ardern said since its creation she had believed in the prize's "power to encourage and spread not only the innovation we desperately need, but also optimism".
The former prime minister of New Zealand attended the Earthshot Prize Innovation Summit in New York in September 2022 where she spoke on behalf of Prince William, who had stayed in the UK following the death of the late Queen Elizabeth.
Chair of the board of trustees Christiana Figueres said she was "thrilled" to welcome Ms Ardern and had long been inspired by her "work as a catalysing force in the effort to combat climate change".
Since her resignation, Ms Ardern is also due to serve as an unpaid special envoy for the Christchurch Call, which she co-founded in 2019 to bring technology companies and countries together to combat extremism, according to Reuters news agency.
Prince William and Ms Ardern have met on numerous occasions, including when the former leader of New Zealand visited Kensington Palace during a visit to Britain in July last year.
Alongside his wife Catherine, Prince William also visited New Zealand in 2019 where he met survivors of the Christchurch mosque attacks.