The Family Bithyniidae widely distributed in the Balkans and is represented by two genera in Europe: Bithynia and Pseudobithynia. More recent data about the Bithyniidae shows that the majority of family members are distributed in the Balkans and especially Greece. Seven species of Bithynia but only one of Pseudobithynia is currently known from Turkey. This study is aimed at describing a new species, Pseudobithynia yildirimi n. sp. from the Biga Peninsula in northwestern Turkey. Pseudobithynia yildirimi n. sp. is compared to related taxa in Turkey, as well as in Greece and neighbouring Iran. The penis variability and status of pseudo-maleness in P. yildirimi were also investigated.