The new mayfly species Prosopistoma orhanelicum sp. n. (Ephemeroptera: Prosopistomatidae) was collected from Orhaneli stream, northwestern Anatolia, Turkey. The main diagnostic larval characters are described and compared with two species which are found in nearby geographic regions. The larvae of P. orhanelicum sp. n. are differentiated by the number of antennal segments, antennal segment 2/3-5 ratio, maxillary palp seg 1/seg 2 ratio, width/length ratio of the head and posterolateral projections on abdominal segments 7-9.