Ms Ardern made the announcement on Facebook, saying she is expecting a child with her partner, television host Clarke Gayford.
Ardern said she first knew of her pregnancy on October 13, in the midst of talks to form a coalition government after an inconclusive general election in September. This year we'll join the many parents who wear two hats.
Ardern said deputy Prime Minister Winston Peters would serve as acting Prime Minister for six weeks after her baby was born.
She posted a photo of two fishing hooks, with a smaller hook inside one of them.
Ardern said she was still focused on the job of being prime minister.
"I think it's fair to say that this will be a wee one that a village will raise, but we couldn't be more excited". "I know there will be lots of questions, and we'll answer all of them (I can assure you we have a plan all ready to go!) But for now, bring on 2018", the statement finished.
The prime minister waited until now to share the news with New Zealanders to ensure the baby was healthy and well.
The charismatic leader enjoyed a meteoric rise previous year, winning office just months after taking the helm of the centre-left Labour Party. Ardern said she will be "contactable and available throughout the six week period when needed".
The Australian Prime Minister called Ardern shortly after the news broke. Pakistan's Benazir Bhutto gave birth while in office in 1990.
The PM announced today she is pregnant. I'm sure you'll be equally as great at being a mum as what you are as the Prime Minister!'