Sidirokastro (Greek: Σιδηρόκαστρο; Bulgarian Валовища Valovišta; Turkish: Demirhisar) is a town and a former municipality in the Serres regional unit, Greece.

It has a number of tourist sights, such as the medieval stone castle, Byzantine ruins, and natural spas.

Sidirokastro is located 25 km to the northwest of the town of Serres, between the Vrontous and Angistro mountains (to the north) and the river Strymonas (to the west).

Sidirokastro took in refugees from Melnik in 1913; from East Thrace (European Turkey) after the 1922 onslaught that followed the Greco-Turkish Wars in Asia Minor; from Pontus, Vlachs and people from all over Greece.

[8] In 1912, Sidirokastro was captured by the Bulgarians under general Georgi Todorov, but some months later it came under Greek control when the Balkan Wars ended.

Sidirokastro is situated on the European route E79; the main road from northern Greece (Thessaloniki) to Bulgaria.