The Cuban-American pop star, 22, appeared on Greg James’s Radio 1 show and confessed to putting the pencil in her mother’s bag during a visit last month.
Cabello, whose hits include Havana and Senorita, apologised for the theft, saying: “So I’m sorry. I still have it. I’m sorry William, and I’m sorry Kate. I honestly couldn’t sleep last night. I had to get it off my chest.”
The royal couple have now responded, with a tweet from the official Kensington Palace Twitter account replying to a video of Cabello’s apology with the two eyes emoji.
👀— Kensington Palace (@KensingtonRoyal)November 26, 2019
Cabello, who is in a relationship with the Canadian singer-songwriter Shawn Mendes, visited the palace alongside James for the BBC Radio 1 Teen Heroes event in October.
Recalling the pencil theft during her Radio 1 interview, she jokingly told James: “Don’t tell anybody what my mother and I did.”
She said the presenter had “triple-doggy-dared” her to take the writing instrument.
Cabello is not the first celebrity to have pinched an item from a royal household.
Olivia Colman, currently playing the Queen in Netflix’s regal drama The Crown, revealed her husband swiped a toilet roll when the couple were attending a charity event at Buckingham Palace.
Spice Girl Emma Bunton admitted to taking a sign for the ladies’ toilets on a visit to the palace during a concert for the Queen’s Golden Jubilee in 2002.
Additional reporting by Press Association.