Rtanj (Serbian: Ртањ, pronounced [r̩̂ːtaɲ]) is a mountain situated in eastern Serbia, approximately 200 km southeast of Belgrade, between towns of Boljevac on north and Sokobanja on south. It belongs to the Serbian Carpathians. Its highest peak is Šiljak (pronounced [ʃǐːʎak]) (1,565 m), a natural phenomenon of karst terrain.