The heir to Queen Elizabeth II chatted to visitors while he toured the castle, his mother's weekend home west of London, with a group of travel executives.
"That was quite a shock," said Stan Axsmith, from Minnesota, after he had his picture taken with the prince. "I didn't think my wife would believe me, so I asked for a photograph."
Honeymooners Darold and Cindy Lee, from Seattle, Washington, said posing with the royal personage had made their vacation.
Stephen Dowd, a British travel executive who accompanied the prince, said British tourism was still suffering from the downturn in air travel that followed the Sept. 11, 2001 terrorist attacks on the United States and the effects of recent Iraq war.
"In particular, visitors from North America are staying away. We estimate that the inbound tourism market could be down by between 15 percent and 20 percent, representing a loss of up to $3.2 billion,'' said Dowd, chief executive of the British Incoming Tour Operators' Association.
"We are delighted that the royal family are offering their support."
Officials said royal attractions are bucking the trend and receiving more visitors than ever.
"The number of visitors to Windsor Castle rose by nearly 10 percent last financial year to about 963,000," said Frances Dunkels of the Royal Collection, which runs the royal art collections.
"Of course, the queen's Golden Jubilee (in 2002) helped to boost numbers, but there is a general upward trend. All royal sites are up by about 30 percent."