In a post on her Instagram account, the 37-year-old wrote:
"And we thought 2017 was a big year! Clarke and I are really excited that in June our team will expand from two to three, and that we'll be joining the many parents out there who wear two hats. I'll be Prime Minister AND a mum, and Clarke will be "first man of fishing" and stay at home dad.
"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."
Winston Peters will be acting Prime Minister for six weeks after she has her baby in June.
"Mr Peters and I have a great relationship, and I know that together we'll make this period work. I will make arrangements for appropriate Ministers to act in my other portfolios over the six weeks I am away from Parliament.
"At the end of my leave I will resume all Prime Ministerial duties."
Ms Ardern said they first knew of the pregnancy in mid-October, "but as many couples do in the early stages, we kept it to ourselves".
Last year, in the space of a few short months, the passionate politician – whose sole reason for joining the Labour Party at 17 was to tackle child poverty – rose from list MP to leader of the Labour party, and then Prime Minister of New Zealand.
She now can look forward to another big role in 2018 - motherhood!
Ardern created headlines around the world in 2017 when she took TV presenter Mark Richardson to task over him asking her her baby plans.
She said it was "totally unacceptable" to say a woman should have to answer that question in the workplace.
"It is a woman's decision about when they choose to have children, it should not predetermine whether or not they are given a job, or have job opportunities."