Ardern was the second prime minister in the world to give birth while in office, after Pakistan's Benazir Bhutto in 1990.
She said she was looking forward to spending more time with her daughter Neve, who is due to start school later this year, and finally getting married to her partner, TV personality Clarke Gayford.
Australian Prime Minister Anthony Albanese led international tributes to Ardern, saying she had "shown the world how to lead with intellect and strength".
"She has demonstrated that empathy and insight are powerful leadership qualities," Albanese said.
The US ambassador to New Zealand, Tom Udall, said Ardern was an "incredible world leader".
Source: Agence France-Presse
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