A general election would be held on October 14, she added.
"This summer, I had hoped to find a way to prepare for not just another year, but another term - because that is what this year requires," a visibly emotional Ardern said during the statement. "I have not been able to do that."
Ardern's term will conclude no later than Feb. 7.
Ardern said she believed the New Zealand Labor Party would win the upcoming election and added that a vote to elect the next Labor leader would be held on Sunday.
New Zealand Deputy Prime Minister Grant Robertson, who also serves as finance minister, said in a statement he would not seek to stand as the next Labor leader.