Dr. Ethelwynn Trewavas (5 November 1900 — 16 August 1993) was an ichthyologist at the British Museum of Natural History. She was known for her work on the families Cichlidae and Sciaenidae. She worked with Charles Tate Regan another ichthyologist and taxonomist.
She served as the senior scientist in the Fish Section of the British Museum of Natural History for almost 50 years, and was known internationally as an authority on several diverse groups of fishes. She was best known for her work describing African Rift lake cichlids, but she published extensively on other groups as well. She used laboratory study and extended field trips to research her current areas of study, and often relied on interviews with local people to understand the behaviours, forms and food potential of fishes.
One example of the influence of Regan and Trewavas is that of the 13 currently named genus categories of Lake Malawi mbuna, two are attributed to Regan and six to Trewavas. Of the 41 genera of Haplochromis in the lake, five were described by Regan and 27 by Trewavas, either individually or in partnership with David Eccles.
Later in life she mentored prominent researcher Ad Konings, who has continued many of her areas of study. When her eyesight failed she insisted that he accept her stereo microscope as a gift so he could continue her work with African cichlids.