John Hemsworth Osborne-Day (25 August 1909 – 24 April 1989 ) was a South African marine biologist and invertebrate zoologist who was born in Sussex and who died in Knysna.

[1] During World War 2 he joined the Royal Air Force and become squadron leader in Bomber Command.

He lost a leg after a bombing raid and received the Distinguished Flying Cross (United Kingdom) and bar.

[2] Following the War, John Day returned to the University of Cape Town and in 1947 was appointed head of the Zoology Department, a position he held until his retirement in 1974.

[4] In 1967 John Day published his two volume Monograph on the Polychaeta of Southern Africa,[5][6] and this publication is still a widely used identification tool to the major groups of these common and diverse marine worms.