The subterranean, carnivorous slug Selenochlamys ysbryda sp. nov. is described from a breeding population in an urban garden in Cardiff, Wales, UK. The species was probably introduced recently among garden plants. Selenochlamys is a distinctive and little-known genus of the Trigonochlamydidae, a family endemic to the Caucasus and neighbouring countries, hitherto unrecorded west of Turkey. Cardiff material differs from specimens and descriptions of the sole previously recognised species S. pallida O. Boettger, 1883 in having vestigial eyes, in greater body size, in musculature, in small differences in the genitalia, and in other features. In colour and vestigial eyes, S. ysbryda resembles certain troglobitic (cave-dwelling) molluscs of the Caucasus but may simply be a deeply edaphobitic (soil-dwelling) animal. We describe the living animal and aspects of the behaviour of S. ysbryda, including the consumption of earthworms. In size, morphology and behaviour there are numerous and striking convergences between S. ysbryda and the West Palearctic genus Testacella (Testacellidae), probably indicating a similar ecology. Testacella is known to have been widely spread by man, as have several earthworm-eating terrestrial planarians. In the light of these comparisons, we speculate on whether S. ysbryda is likely to occur elsewhere undetected or spread further.