Postojna (German: Adelsberg, Italian: Postumia) is a town and a municipality in the traditional region of Inner Carniola, 35 kilometers (22 miles) from Trieste, in southwestern Slovenia.
The entire municipality has a typical karstic landscape.
One of Slovenia's major tourist attractions, the Postojna Cave, is located in the municipality. The cave was first described in the 17th century by Janez Vajkard Valvasor, and a further major part of the cave was discovered in 1818. Electric lighting was added in 1872, increasing the cave system's popularity. A second popular tourist attraction, located approximately 9 kilometres from Postojna, is Predjama Castle, a 16th century castle, built in the mouth of another Karst cave.