Polykastro is a town and a former municipality in Kilkis regional unit of Central Macedonia, Greece. Since the 2011 local government reform it is part of the municipality Paionia, of which it is a municipal unit, and the seat. The municipal unit has an area of 312.717 km2, the municipal unit 45.775 km2. The municipal unit of Polykastro has 12,000 inhabitants, and includes Polykastro and 23 villages. It is built near the Axios River, on the road and railway from Thessaloniki to Belgrade. It was formerly known as Karasuli (Turkish), Mavrosuli or Rugunovec (Bulgarian: Ругуновец).