Allen Leech — who stars as Tom Branson in the hit period drama — revealed the news of the show's royal supporters during an interview on Andy Cohen's Watch What Happens Live!.
"Obviously he's now got a lot of down-time, so she's gone, 'I'm watching this and you're watching it with me'," he joked.
Allen appeared on the US chat show alongside cast mates Rob James-Collier, Laura Carmichael and Lesley Nicol.
When one viewer called in to ask if any of them had met the royal family, Allen responded, "I was actually at an event last week with Prince William, now the Duke of Cambridge. So yeah, I met him last week… shook his hand.
"He actually said, 'I'm a big fan of the show. Only now that my wife's had a baby'," he revealed.
William and Kate are not the only famous fans of the show. Last year it was revealed that Michelle Obama loves Downton, and begged ITV bosses to ship her over series three ahead of its official US launch.
A source on the show told The Sun, "We know it's very popular in America as it's so quintessentially British. We'd heard the Obamas liked to watch it but we were very surprised when someone from the First Lady's office got in touch asking for the series.
"They were very polite and we were more than happy to sort out some DVDs for them to send over. Obviously we hope they keep the spoilers to themselves as our US fans may not know about... well, you know what!"