Postojna is a town in the traditional region of Inner Carniola, 35 kilometers (22 mi) from Trieste, in southwestern Slovenia. It is the seat of the Municipality of Postojna.

The area is known to have been populated since the paleolithic era due to the discovery of a cave settlement near the town of Postojna called Betal Rock Shelter (Betalov spodmol). The town lies on the Pivka River. Written sources first mention the settlement in the 13th century and in 1432 it became a borough. It was proclaimed a town in 1909.

From the late Middle Ages, it was part of the Duchy of Carniola and hence of the Habsburg Monarchy. It was under Italian rule between 1918 and 1943 (nominally to 1947) and was part of the province of Trieste as Postumia.