William Swainson (October 8, 1789 - December 6, 1855), was an English ornithologist and artist.

Swainson was born in St. Mary Newington, London. At the age of fourteen he became a customs clerk in Liverpool. He developed an interest in natural history by studying his fathers shell and insect collections. He was drafted into the army and sent to Sicily, but was forced to retire due to ill health.

Swainson travelled in Brazil from 1816 to 1818, returning to England with a collection of over 20,000 insects, 1,200 species of plants, drawings of 120 species of fish, and about 760 bird skins. His friend William Elford Leach encouraged him to experiment with lithography for his book Zoological Illustrations (1820-23).

When Leach was forced to resign from the British Museum due to ill health, Swainson applied to replace him, but the post was given to John George Children. Swainson continued with his writing, the most influential of which was the second volume of Fauna Boreali-Americana (1831) which he co-authored with John Richardson. He also produced a second series of Zoological Illustrations (1832-33), three volumes of Jardine's Naturalist's Library, and eleven volumes of Lardner's Cabinet Cyclopedia.

In 1841 he emigrated to New Zealand to become a farmer, but failed partly due to the opposition from local Maoris. In 1851 he sailed to Sydney and he took the post of Botanical Surveyor with the Victoria Government. This also failed, apparently due to his lack of knowledge of botany. He returned to New Zealand in 1855, where he died.

Swainson's friend John James Audubon named Swainson's Warbler after him, Charles Lucien Bonaparte named Swainson's Hawk for him, and Swainson's Thrush was named for him by Thomas Nuttall.