Socerb Castle, Medieval castle on Karst Edge cliff near Koper, Slovenia.

Socerb Castle is a fortified structure perched on a limestone cliff at the Karst Edge near Koper, sitting about 350 meters above sea level. It contains preserved defensive walls, former residential rooms, and a dining space housed in what was once the main hall.

Socerb Castle was first mentioned in 948 as a possession of the Diocese of Trieste. In 1463 it was granted to the Venetian Ducaini family, and in 1521 Emperor Charles V enfeoffed it to the Carniolan noble Nikolaj II Ravbar. A lightning strike set it on fire in 1780, after which it fell into ruin; in 1924–25 Demetrius de Economo had part of the perimeter wall restored.

The name Socerb comes from Saint Servulus, the young hermit said to have lived in the nearby cave for two years before his execution in 283 or 284. Until World War II, a Mass was held at the cave every 24 May. The castle itself was visited, drawn and described by the historian Janez Vajkard Valvasor in 1678.

The castle stands on a rock over 350 m above the Zaule plain, a short distance from the Italian-Slovenian border; the entrance to the Sacred Cave is 150 m away. Only part of the perimeter wall was restored in 1924/25, so the ruins are modest and the site is reached on foot. The cave is 44 m deep and 200 m long.

The Sacred Cave of Saint Servulus lies about 150 metres from the castle, not beneath it. It is 44 metres deep and 200 metres long; the young saint lived there as a hermit for two years after converting to Christianity, and was executed in 283 or 284. Mass was held in the cave every 24 May until World War II.