John Cay FRSE PRSSA (1790–1865) was a Scottish advocate, pioneer photographer and antiquarian. He served as the Sheriff of Linlithgowshire 1822–65. He was the maternal uncle of James Clerk Maxwell. He was an original member of the Edinburgh Calotype Club one of the world’s first photographic clubs (1843) and a keen early photographer alongside his friend Sir David Brewster and the Edinburgh pioneers David Octavius Hill and Robert Adamson. He made very early photographic presentations to the Royal Scottish Society of Arts.