The Macestus Bridge or Bridge of Sultançayır was a Roman segmental arched bridge across the Macestus River (Turkish: Simav or Susurluk Çayı) at Balıkesir, in the northwestern part of modern-day Turkey. Its flattened arches, slender piers and the hollow chamber system documented the progress made in late antique bridge building. A first cursory investigation of the 234 m long structure was conducted in the early 20th century, but since then its existence has been largely neglected by scholars.