The Duke of Cambridge was seen wondering around Rochester Row, in Westminster, wearing a red Big Issue hat and vest.
Matthew Gardner, a retired Met Police officer, shared a photo of the Prince on his LinkedIn account.
The former cop revealed that his brother-in-law cottoned on to the Duke as he walked across the road, and even managed to get a pic with him, reports the BBC.
Gardner wrote on his LinkedIn: "My bother in law was in London today and saw a celebrity, so he took a photo at a distance.
"The celebrity saw the ‘covert surveillance’ effort and crossed the road to investigate further.
"What an honour to have a private moment with our future King who was humble and working quietly in the background, helping the most needy.
"These ‘silent gestures’ often go unrecognised.
"The finale to this unique occasion was when Prince William asked my brother in law if he wanted to buy the Big Issue, to which he replied 'I have no change'.
"At this point William produced a mobile card machine… you cannot teach that! Priceless, or should I say ‘Princely’."
One user commented: "How beautiful is that!"
While a second added: "Wow. What a guy. That’s priceless."
Neil Kramer, a London taxi driver, was also able to get a pic with the Duke.
After sharing the post on his Insta, he captioned it: "Hi I'm William would you like to buy a copy of the Big Issue."
In the comments, someone asked the cabbie if he bought a copy, to which he replied: "Yes of course I did."
Another person wrote: "What an amazing man. I'm no royalist but this is just, well, wow!"
One person, who's clearly a big fan of the Royal family, added: "This has really hit me seeing our future king silently helping.
"Making what seems a small difference without recognition. But will have a profound effect on the people who he's helping.
"Imagine if we all did something like this? Amazing!"
The Big Issue says it aims 'to offer vulnerable people the opportunity to work their way out poverty'.
Via their website, they explain: "Big Issue sellers buy every copy of the magazine upfront for 50 percent of the cover price.
"The Big Issue Foundation, our charitable arm, works exclusively with Big Issue vendors, who have already demonstrated their commitment to making positive changes in their lives."
LADbible have contacted Big Issue for comment.