Castelmur Castle is a castle in the village of Bondo in the municipality of Bregaglia of the Canton of Graubünden in Switzerland. It is a Swiss heritage site of national significance. The fortifications at Castelmur may be, after the Three Castles of Bellinzona, the most important example of medieval valley fortifications in modern Switzerland.

The castle site has been occupied and fortified since at least the Roman era. The important trade road over the Septimer Pass runs through the Val Bregaglia. At Bondo the valley narrows leaving a narrow passage between the Maira river and a large rocky outcropping, making the castle site an ideal location for a customs station and fortification.