The helicoid genus Thiessea comprises 16 species, of which 15 are distributed in the Aegean and the eastern coastal part of Greek mainland and one species is present in the western coast of Turkey. Two species were known only from shells, T. pieperi (Kasos island, southern Aegean) and T. valkanovi (Thasos island, northern Aegean). The thorough survey of Kasos for land gastropods gave us the opportunity to discover live T. pieperi. The species was found aestivating in limestone crevices on Mount Prionas. Based on the fresh material collected, we give new description of the shell and, for the first time, we describe the species’ reproductive system and compare it with its congenerics. This find reinforces the taxonomic status of the species and furthermore corroborates that there are few extinctions of land gastropods during Holocene in the Aegean archipelago.