President Obama joked Britain's Prince George committed a "clear breach in protocol" when he met him in a bathrobe.
The President of the United States was visiting London when he was granted an audience with the two-year-old tot - the son of the Duke and Duchess of Cambridge - and quipped it was a "slap in the face" when he opted for casual attire at the meeting.
Speaking at the White House Correspondents' Dinner, he said: "Even some foreign leaders, they've been looking ahead, anticipating my departure. Last week, Prince George showed up to our meeting in his bathrobe. That was a slap in the face. A clear breach in protocol.
"Although while in England I did have lunch with Her Majesty, the Queen, took in a performance of Shakespeare, hit the links with David Cameron - just in case anybody is still debating whether I'm black enough, I think that settles the debate."
Meanwhile, elsewhere in his speech, President Obama continued to crack jokes about his wife First Lady Michelle Obama and celebrity guests including Kendall Jenner and Dame Helen Mirren.
He said: "My brilliant and beautiful wife, Michelle, is here tonight. She looks so happy to be here. That's called practice - it's like learning to do three-minute planks. She makes it look easy now ...
"Kendall Jenner is also here. And we had a chance to meet her backstage - she seems like a very nice young woman. I'm not exactly sure what she does, but I am told that my Twitter mentions are about to go through the roof.
"Helen Mirren is here tonight. I don't even have a joke here. I just think Helen Mirren is awesome. She's awesome."