Ms Ardern said she and her partner, Clarke Gayford, were expecting their child in June, after which she planned to take a six-week break.
“And we thought 2017 was a big year!” she wrote on Instagram.
Ms Ardern, 37, became New Zealand’s youngest prime minister since 1856 when she formed a centre-left coalition in October.
She was flooded with support after making the announcement on her social media profiles on Friday.
Ms Ardern’s Labour Party came second in September’s election, where no party was able to secure a majority.
She formed a government with the backing of Winston Peters, leader of the small New Zealand First party.
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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.
“As is the case when I am overseas, Mr Peters will act as prime minister, working with my office while staying in touch with me,” Ms Ardern said in a statement reported by the New Zealand Herald on Friday.
“I fully intend to be contactable and available throughout the six-week period when needed.”
Mr Gayford would be a “stay-at-home” dad, she said.
Two of New Zealand’s former prime ministers were among the first to offer congratulations.
Wishing @jacindaardern & @NZClarke all the best as they expect their 1st child in June: a super busy year coming up & much to look forward to. Every #woman should have the choice of combining family & career. https://t.co/Ma6B6OGXJe
— Helen Clark (@HelenClarkNZ) January 18, 2018
— Bill English (@RtHonBEnglish) January 18, 2018
Ms Ardern took over the Labour leadership in July. She later said that she had an hour’s notice that Andrew Little was about to stand down.
On her first day as opposition leader, Ms Ardern was controversially asked by a TV talk show host whether she had made “a choice between having babies and having a career”.
Ms Ardern replied that such a question was “totally unacceptable in 2017”.
“It is a woman’s decision about when they choose to have children and it should not predetermine whether or not they are given a job or have job opportunities,” she said.