Prince George became the star of the Kensington Palace party after greeting Michelle and Barack Obama in his dressing gown and slippers.
The Obamas arrived at the palace for an informal dinner with Prince William, Kate Middleton and Prince Harry.
But it was the two-year-old prince who stole the limelight after pictures emerged of George shaking hands with Obama wearing his nightwear - a pair of freshly-pressed blue gingham pajamas, monogramed dressing gown and a pair of slippers with aeroplanes on.
The third in line to the throne was allowed to stay up for a few minutes by his parents, the Duke and Duchess of Cambridge, so the two-year-old could thank the couple for a rocking horse and stuffed toy they had previously given him as gifts.
It is the first time that any of the young royals have entertained a head of state privately and the most significant event they have ever hosted at Kensington Palace.