Polykastro is a town located in the Kilkis regional unit of Central Macedonia, Greece. Since the local government reform of 2011, it serves as the seat of the municipal unit of Polykastro within the larger municipality of Paionia. The town is situated near the Axios River along the road and railway connecting Thessaloniki to Belgrade.
The municipal unit of Polykastro covers an area of 45.775 km2 and has a population of approximately 12,000 inhabitants. It encompasses the town itself along with 23 villages. Historically, the location was known by various names, including Karasuli (Turkish), Mavrosuli, or Rugunovec.