William Dyce Cay, MICE FRSE (1837 or 1838 – 13 December 1925) was a Scottish civil engineer. He was responsible for the majority of late 19th century works to Aberdeen harbour. He was described by his cousin, James Clerk Maxwell, as a "watery engineer".
He was born the son of Robert Dundas Cay, an Edinburgh lawyer, and Isabella Dyce (1811–1852).
In 1844 the family moved to Hong Kong following his father's appointment as Registrar to the Supreme Court of that city. His mother died as the result of injuries from a carriage accident in 1852 and is buried in the Happy Valley Cemetery in Victoria, Hong Kong.
William returned to Edinburgh soon after this, to study Mathematics at Edinburgh University, winning the prestigious Straiton Gold Medal in 1856. He then moved to Belfast to serve as an apprentice engineer under Lord Kelvin's brother, James Thomson. On completion of his training in 1858 he began to specialise in harbour design, and spent the bulk of his working life improving Aberdeen's docks and harbours.
He was elected a Fellow of the Royal Society of Edinburgh in 1882. In 1890 he obtained a house on Blackford Road in Edinburgh.
He died at Folkestone, Kent on 13 December 1925.
- Castle Douglas to Portpatrick Railway (as assistant to Benjamin Blyth) (1861)
- Glenlair Bridge near Corsock, Dumfriesshire (1861) built for his cousin, James Clerk Maxwell who owned Glenlair House
- Cluny Harbour, Buckie (1873)
- Southern Breakwater, Aberdeen Harbour (1874)
- Lerwick Harbour extensions and improvements (1881)
- "Visit by Panel for Historic Engineering Works | Glenlair". glenlair.org.uk. Retrieved 2015-09-25.
His uncles (his mother's brothers) were William Dyce and Prof Robert Dyce.
His cousin (through his father's sister) was James Clerk Maxwell. They were close friends and Cay served as Maxwell's best man upon his marriage.