New York City Mayor Zohran Mamdani said he encourages Britain's King Charles to return the Koh-i-Noor Diamond, with his comments coming during the monarch's ongoing US visit.
"If I were to speak to the king separately from that, I would probably encourage him to return the Koh-i-Noor Diamond," Mr Mamdani, who is Indian American, said when asked at a press conference hours before a ceremony that commemorated victims of the September 11 attacks.
Later in the day, the king spoke with Mr Mamdani at the ceremony.
Buckingham Palace declined to comment.
Watch: New York City mayor calls on Britain's King Charles to return Koh-i-Noor Diamond
Mr Mamdani's office did not respond to a request to comment on whether the mayor brought up the issue with the king.
India has previously repeatedly demanded that Britain return the 105-carat diamond.
Britain's then colonial governor-general of India arranged for the huge diamond to be presented to Queen Victoria in 1850 after the East India Company had annexed the region of Punjab in 1849 and taken the diamond from a deposed Indian leader.
Charles commemorated victims of the September 11 attack on New York City, laying a floral bouquet at the memorial where the World Trade Center's twin towers once stood.
India received independence from British rule in 1947. The British colonisation of India and widespread atrocities committed against Indians during that period remain sensitive issues in the country.
India has previously said the diamond was a "valued piece of art with strong roots in our nation's history."
The diamond's possession by the British is seen by many Indians as a symbol of colonial atrocities during British rule.
The diamond has been previously owned by India's Mughal emperors, shahs of Iran, emirs of Afghanistan, and Sikh maharajas, according to the Historic Royal Palaces charity.
