The shell-less gastropod Entocolax olgae sp. n., an endoparasite of the holothurian Myriotrochus rinkii Steenstrup, 1851, is described from two localities in the eastern part of the Laptev Sea. Entocolax olgae has a deep longitudinal fold in the intestine, a unique feature differentiating it from all other known members of the genus. There are also some differences in the reproductive system and siphon structure. Taxonomically significant morphological features of other Entocolax Voigt, 1888 species known from Eurasian seas are also reviewed.