English said that after 27 years in parliament he decided during his summer vacation that he wanted a break from politics.
"This is more about myself and my family having spent most of my adult life, all of their lives, with the demands of politics," the 56-year-old told reporters.
"I want the opportunity to start again on a different life."
A staunchly Catholic conservative, English served as prime minister for just under a year before being replaced by centre-left leader Jacinda Ardern in October last year.
After eight years as finance minister, he was seen as a dour replacement for the charismatic John Key but almost derailed the "Jacinda-mania" bandwagon in last year's vote.
He outpolled Ardern in the popular vote but she forged an alliance with the Greens and maverick lawmaker Winston Peters to form a coalition government.
English said he felt it was time for his National Party for anoint a new team to take to the next election, although he did not nominate a preferred successor.
Ardern paid tribute to him.
"He is a man of clear convictions who has always had a genuine concern for the well-being of New Zealanders," she said.
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