William John Swainson (8 October 1789 – 6 December 1855), was an English ornithologist, malacologist, conchologist, entomologist and artist.

Swainson was born in Dover Place, St Mary Newington, London, the eldest son of John Timothy Swainson the Second (1756-1824), an original fellow of the Linnean Society. He was cousin of the amateur botanist Isaac Swainson. His father's family originated in Lancashire, and both grandfather and father held high posts in Her Majesty's Customs, the father becoming Collector at Liverpool.

William, whose formal education was curtailed because of an impediment in his speech, joined the Liverpool Customs as a junior clerk at the age of 14. He joined the Army Commissariat and toured Malta and Sicily. He studied the ichthyology of western Sicily and in 1815, was forced by ill health to return to England where he subsequently retired on half pay. William followed in his father's footsteps to become a fellow of the Linnean Society in 1815.

In 1806 he accompanied the English explorer Henry Koster to Brazil. Koster had lived in Brazil for some years and had become famous for his book Travels in Brazil (1816). There he met Dr Grigori Ivanovitch Langsdorff, also an explorer of Brazil, and Russian Consul General. They did not spend a long time on shore because of a revolution, but Swainson returned to England in 1818 in his words "a bee loaded with honey", with a collection of over 20,000 insects, 1,200 species of plants, drawings of 120 species of fish, and about 760 bird skins.

Apart from the common and scientific names of many species, it is for the quality of his illustrations that he is best remembered.