Prince William, 42, who is known as the Duke of Cornwall in the county, was joined by his aunt, Sophie, 60, at the event.
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The pair were all smiles as they arrived at the Royal Cornwall Showground as they took selfies with well-wishers and toured the different stalls, which included local delicacies and tipples.
At the gin stand, William quipped: "I can see what today's going to be like."
After the pair both tried some of the spirit, he caught the Duchess going for another sip and joked: "You're not allowed to finish it. You've got to watch her."
Sophie responded: "He's learnt from the best."
William responded: "I've been caught out with whisky a few too many times. I think I can do that and then oh no I couldn't."
William is patron of The Royal Cornwall Agricultural Association while Sophie is patron of The Association of Agricultural Show Organisations (ASAO) and Vice President of the Royal Cornwall Agricultural Association.
The show, which dates back to 1793, showcases the very best in Cornish heritage and tradition through agriculture, food and drink, entertainment and competition.
See all of the best photos from their joint outing.
Prince William and Sophie appeared to exchange a joke as they arrived at the Royal Cornwall Show together, with hundreds of well-wishers gathering to catch a glimpse of the pair.
William was dressed in a checked blazer, a blue shirt, dark trousers and a blue tie for the outing, while Sophie opted for a brown suede skirt, a light pink blouse and an olive-green jacket.
The pair then visited the Royal Cornwall Food and Farming Pavilion which showcases dozens of the best artisan food and drink producers from across Cornwall, including Cornish cheeses, jams and preserves, as well as gins and other spirits.
William and Sophie were given pasties by Ferguson Miller, 45, Amelia Jones, 25, and Alan Anderson, 69, of Anns Pasties.
William said: "Ah the pasties arrived! They're hot as well. There's been a few people I passed on the way here with their pasties ready to go. Too good. I'm going to save this for my lunch. Trying to talk and eat is a nightmare."
The pair looked like they were having a lot of fun while touring the different stalls and sampling the goods on offer.
William asked about the "zero" on the bottle of cider, and when told that it meant zero alcohol he put the cup down and said: "Oh no, no, no. I need original. Someone bring out the big guns."
Before departing, the Prince and Duchess posed for a group picture with staff at the show.
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