The Prime Minister is engaged to her longtime partner, Clarke Gayford.
The couple, who are parents to 10-month-old daughter Neve, became engaged over Easter, a spokesman for Jacinda Ardern said.
The couple are understood to have spent Easter in Mahia at Clarke Gayford's family bach.
Ardern's press secretary said she has been wearing a ring since Easter but it has been on a middle finger, not the traditional ring finger.
It is not yet known whether Ardern or Gayford popped the question.
But the Prime Minister has previously said she wouldn't be letting the 41-year-old off the hook by asking him to marry her.
Ardern laughed raucously when asked whether a marriage proposal on her part might be in the couple's near future in a BBC interview in January.
During a wide-ranging interview, host Victoria Derbyshire asked Ardern if she could imagine asking Gayford to marry her.
"I would not ask, no. No."
When Derbyshire pointed out she was a feminist, Ardern replied: "Absolutely I'm a feminist. But I want to put him through the pain and torture of having to agonise about that question himself.
"No, that's letting him off the hook. Absolutely not," she said, laughing.
A ring, thought to be diamond, was spotted by reporters on the PM's hand at a ceremony at Pike River on the West Coast today,
Ardern, 38, has previously quashed speculation she and Gayford were engaged.
In December 2017, two months after she became New Zealand's 40th Prime Minister, Ardern addressed speculation about a ring seen on a finger of her left hand.
"I thought I would upfront address the speculation in the gallery this afternoon over my Facebook Live and the issue of where my ring was placed during that Facebook live," Ardern told media at a post-cabinet press conference.
"I'm happy to confirm that I have eczema on my left hand, which causes me to rotate where I wear my beautiful onyx ring. So no I am not engaged", she said at the time.
"I do however suffer from a small skin condition, which is not very romantic. Glad to have cleared that up."
The researchers aim to create early detection tests, improving survival rates.