John Thomas Young Gilroy was an English artist and illustrator, best known for his advertising posters for Guinness, the Irish stout. He signed many of his works, simply, "Gilroy".

Born in Whitley Bay, Northumberland, England, Gilroy attended Durham University until his studies were interrupted by World War I, during which he served with the Royal Field Artillery. He resumed studying at the Royal College of Art in London, where he remained as a teacher. He taught at Camberwell College of Arts.

In 1925, he gained employment at S.H. Benson's advertising agency, where he created the iconic advertisement art for Guinness featuring the Zoo Keeper and animals enjoying Guinness. He worked with Dorothy L. Sayers. He created cover designs for the Radio Times, most famously, in 1936, one depicting a laughing cat. He was also an accomplished portrait painter, numbering royalty, politicians, actors and many others amongst his sitters. He worked in his large studio at 10 Holland Park, London, the former home and studio of Sir Bernard Partridge. He was a long-standing and much loved member of the Garrick Club, where he was created a Life Member and Chairman of the Works of Art Committee 1970–1975. He was awarded and Honorary MA by Newcastle University in 1975, and was made a Freeman of the City of London in 1981

John Gilroy died at Guildford on 11 April 1985.