Castle Wemyss was a large mansion in Wemyss Bay, Scotland.
It stood on the southern shore of the Firth of Clyde at Wemyss Point, where the firth turns southwards. It was built around 1850 for Charles Wilsone Brown, a property developer who had plans to develop the land around Wemyss Bay, and who by 1855 had increased the number of villas from four to thirty six. These villas earned the nickname 'Little Glasgow' because they were let to wealthy Glasgow merchants.
Wilsone Brown sold the mansion to Sir John Burns (later Baron Inverclyde) in 1860. Burns commissioned the architect Robert William Billings to remodel the house in the Scottish baronial style, expanding the original structure by adding a new floor, new wings and a clock tower to the south-east corner.
Castle Wemyss became a fashionable destination for many well-known visitors, including Lord Shaftesbury, Anthony Trollope, General Sherman, Henry Morton Stanley, Peter II of Yugoslavia, Emperor Haile Selassie and members of the British Royal Family.
It is reputed that Trollope wrote part of Barchester Towers whilst at Wemyss Bay, and that 'Portray Castle' in The Eustace Diamonds was based on Castle Wemyss. Whether this is true or not, Trollope places Portray in a similar geographical location, with a description which is very like that of the castle and its grounds. Trollope did however include the real Castle Wemyss in his travelogue How the 'Mastiffs' Went to Iceland, a record of a trip from the Clyde to Iceland in June and July 1878.