William Sweetland Dallas (1824–1890) was a British zoologist. His son was James Dallas.
He curated collections at the British Museum and the Yorkshire Philosophical Society, and was editor of the Popular Science Review. Notably, he translated Facts and Arguments for Darwin by German biologist Fritz Müller into English. He also translated Karl Theodor Ernst von Siebold's Wahre Parthenogenesis bei Schmetterlingen und Bienen (1856) into English as On a true parthenogenesis in moths and bees and created the index for Charles Darwin's The Variation of Animals and Plants under Domestication.
He died at Burlington House, Piccadilly on 29 May 1890 and was buried at West Norwood Cemetery.