John Hay (25 November 1873 – 21 April 1959) was a British cardiologist. He was born in Birkenhead, Lancashire, the son of a Scottish architect and educated at the Liverpool Institute and the Victoria University of Manchester, qualifying M.B. in 1896. From 1900 to 1903 he was medical tutor and registrar at the Liverpool Royal Infirmary. In 1905 he identified a form of second degree AV block. In 1907 he was appointed Assistant Physician and set up the first specialised heart department in the north of England.