Ustra (Bulgarian: Устра) is a fortress in the eastern Rhodope Mountains in southern Bulgaria. Its ruins lie 4 kilometres (2.5 mi) southwest of the village of Ustren situated on a hill at approximately 800 metres (2,600 ft) above sea level.^[1]

Ustra
Bulgarian: Устра
Kardzhali Province, Bulgaria
The fortress walls from the outside.
Ustra
Coordinates	41°28′13″N 25°13′33″E / 41.47028°N 25.22583°E / 41.47028; 25.22583
Site information
Condition	In ruins
Site history
Events	Byzantine–Bulgarian Wars

The fortress was built in the 10th century AD to protect an important trade route. It was taken by the armies of Simeon the Great (893-927), but after his death, it was among the lands given back to Byzantium in return for recognition of the imperial title of the Bulgarian rulers. Between the 12th and 14th centuries, it frequently changed hands between the two empires, but the Byzantines held it most of the time.

The ruins are located 1.5 kilometres (0.93 mi) from the road between Ustren and Zlatograd. There is a hut of the same name nearby. The height of the surviving walls is 10 metres (33 ft).