When Catherine sat to talk to Arthur Jones in Arromanches-les-Bains, Normandy, he asked her: "Is it okay to kiss a Princess?" Laughing, she replied: "Of course it is."
Mr Jones, from the 7th Armoured Division, gave her a kiss on the cheek, watched by Prince William.
Afterwards, Mr Jones, from Wolverhampton, said: "As the Prince left he said to me, 'Were you chatting up my wife?' I told him I only gave her a kiss. William laughed but I'm chuffed I've chatted up a Princess."
The royal couple also attended a rousing commemoration near Gold Beach in Arromanches, where British troops went ashore on D-Day.
William told the ex-servicemen: "As nations - British, French and others - there can be no stronger tie than recollection of what the people of Normandy and thousands of young Allied soldiers, sailors and airmen endured together 70 years ago, this day."