Afyonkarahisar (Turkish for the black opium castle) is a city in western Turkey, also known simply as Afyon (i.e. opium) or as Karahisar-i Sahip. Older spellings include Afium-Kara-hissar and Afyon Karahisar. It is the capital of Afyon province. It is located 250 km south-west of Ankara along the Akar River at an elevation of 1 034 meters. It has a population of 128 516 and is an important railroad junction between Izmir, Konya, Ankara and Istanbul. The region is traditionally the main producer of opium in Turkey, thereby its name.

Afyon was known as Acroënus until the reign of the Byzantine emperor Leo III who after his victory over Arab besiegers in 740 renamed it Nicopolis (Greek for the Victory City). The Seljuk Turks changed its name to Kara Hissar (the black castle) after the ancient fortress situated upon a volcanic rock 201 meters above the town.

It was conquered by the Ottoman Sultan Beyazid I in 1392 but was lost after the invasion of Timur Lenk in 1402. It was recaptured in 1428 or 1429. During the Turkish War of Independence in the early 1920s it was occupied by Greek forces. After 1923 it became a part of the Republic of Turkey.

Apart from the partly ruined fortress which has given the city its name, famous buildings includes the Ulu Camii (Great Mosque) and the Altigöz Bridge, both built by the Seljuks in the 13th century. It is the seat of an Armenian bishop.