Sidirokastro is a town and a former municipality in the Serres regional unit, Greece. Since the 2011 local government reform it is part of the municipality Sintiki, of which it is the seat and a municipal unit. It is built near the fertile valley of the river Strymonas, on the bank of the Krousovitis River. Sidirokastro is situated on the European route E79 and the main road from northern Greece (Thessaloniki) to Bulgaria. It has interesting sights, such as the medieval stone castle, Byzantine ruins, natural spas and beautiful nature.