Ardern was spotted wearing a diamond ring on the middle finger of her left hand, which prompted questions to the prime minister’s office about her status.
Her spokesman then confirmed the couple got engaged over Easter.
Ardern and Gayford had a daughter, Neve Te Aroha, last year.
“I’m very, very lucky,” she told Radio NZ.
“I have a partner who can be there alongside me, who’s taking up a huge part of that joint responsibility because he’s a parent too, he’s not a babysitter.”
She was the second world leader to give birth while in office, after the late Benazir Bhutto – Pakistan’s two-time prime minister.