The palace released a picture of the pair sitting on sofas and chatting at Prince William’s London residence.
A spokesperson for the Duke said the meeting took place earlier on Saturday (2 July).
Ardern is in London for talks with prime minister Boris Johnson, with trade and the war in Ukraine dominating the agenda.
On Friday (1 July), Johnson greeted the politician with a handshake outside the door of No 10 and pointed out that it was her first trip since the two countries signed a fresh free trade deal in February.
The two leaders issued a joint statement following their meeting and pledged that the UK and New Zealand would build “a more secure, sustainable and prosperous future together”.
Together, Johnson and Ardern condemned Russia’s invasion of Ukraine and called for the former’s “immediate withdrawal”. Both leaders also committed to “working harder to support an international [trading] system”.
Ardern previously met William in 2019, when the Duke visited New Zealand and joined her on Anzac Day to pay tribute to the victims of the Christchurch mosque attacks.
The terrorist attack took place in March 2019 and saw white supremacist Brenton Tarrant shoot and kill 51 people at two mosques.
The attack, which was livestreamed, led to New Zealand reforming its gun laws less than a month after it took place.
The country’s parliament quickly voted to ban military-style semi-automatic weapons as well as parts that could be used to build prohibited firearms.
During William’s 2019 visit, he was greeted with a traditional Maori nose press, or hongi, and laid a wreath at the Auckland cenotaph on behalf of the Queen.
He also met with survivors of the attack, including then-five-year-old Alen Alsati and her father Wasseim at the Starship Children’s Hospital. Both father and daughter were injured during the shootings.