The Koh-i-noor – widely called the world's most famous diamond – has been part of the British Crown Jewels since it was acquired from India in the 19th century.
Ahead of a meeting with British King Charles, who is visiting the United States, Zohran Mamdani raised the issue of the 105.6-carat stone.
"If I could talk to the king, I would probably encourage him to return the Koh-i-noor diamond," said the left-wing Democrat.
However, it is unclear whether he did so.
The Koh-i-noor has adorned the crown worn by Queen Mother Elizabeth during her coronation since the 1930s. It is kept in the Tower of London.
"That's where it will stop," writes Zia Yusuf, the domestic policy spokesperson for immigration-critical Reform UK on X, accusing Mamdani of insulting the king.
