William Elford Leach (February 2, 1790 - August 26, 1836) was an English zoologist and marine biologist.

Leach was born in Plymouth, the son of a solicitor. At the age of twelve he went to school in Exeter, studying anatomy and chemistry. By this time he was already collecting marine samples from Plymouth Sound and along the Devon coast.At seventeen he began studying medicine at St Bartholomew's Hospital in London, finishing his qualification at Edinburgh University and St Andrews University.

In 1813 Leach returned to his zoological interests and was employed as assistant librarian in the Zoological Department at the British Museum. He set himself to sorting out the collections, many of which had been neglected since they had been left to the museum by Hans Sloane. During his time there he was made assistant keeper of the natural history department and became an expert on crustaceans and mollusks. He also worked on insects, mammals and birds.

Leach's nomenclature was a little eccentric - he named twenty-seven species after his friend John Cranch, who had collected the species in Africa and later died on HMS Congo. In 1818 he named nine genera after Caroline or anagrams of that name, possibly after his mistress.

In 1821 he suffered a nervous breakdown due to overwork and resigned from the museum in March 1822. His elder sister took him to continental Europe to convalesce, and they travelled through France, Italy and Greece. He died of cholera in the Palazzo San Sebastiano, near Tortona, north of Genoa.

The Leach's Storm-petrel was named after him by Louis Jean Pierre Vieillot. A specimen of this bird had been purchased by Leach on behalf of the British Museum for £5 15s in the sale of the collection of William Bullock in 1819. At the same sale he also bought a Great Auk and an egg for just over £16.

The Blue-winged Kookaburra Dacelo leachii was also named for him.

Leach's written works during his time at the British Museum include the following:

• Zoological Miscellany (1814-1817)
• Monograph on the British Crabs, Lobsters, Prawns and other Crustacea with pendunculated eyes (1815-1817)
• Systematic catalogue of the Specimens of the Indigenous Mammalia and Birds that are preserved at the British Museum (1816)
• Synopsis of the Mollusca of Great Britain (circulated 1820, but not published until 1852).

References

• Barbara and Richard Mearns - Biographies for Birdwatchers ISBN 0124874223