The Simav (Turkish: Simav Çayı) or Susurluk River (Susurluk Çayı) is a river in Anatolian Turkey. Its course is 321 km long and its basin comprises 22 400 km2. It was the classical Macestus (/məˈsɛstəs/; Greek: Μέκεστος, Mékestos). In the 19th century, it was known as the Mikalick.The Simav has its source in Kütahya Province, from which it flows north across the plain of Simav into Balıkesir Province.