Afyonkarahisar (Turkish pronunciation: [ɑfˌjon.kɑˈɾɑhisɑɾ], Turkish: afyon 'poppy, opium', kara 'black', hisar 'fortress') is a major city in western Turkey. It is the administrative centre of Afyonkarahisar Province and Afyonkarahisar District. Its population is 328.319 (2024). Afyon is in the mountainous countryside inland from the Aegean coast, 250 km (155 mi) south-west of Ankara along the Akarçay River. In Turkey, Afyonkarahisar stands out as a capital city of hot springs and spas, an important junction of railway, highway and air traffic in West-Turkey, and the place where independence was won. In addition, Afyonkarahisar is one of Turkey's leading provinces in agriculture, globally renowned for its marble and is the world's largest producer of pharmaceutical opium. In antiquity, the city was called Akroinon and it is the site of Afyonkarahisar Castle, built around 1350 BC.

The name Afyon Kara Hisar literally means opium black fortress in Turkish, since opium was widely grown here and there is a castle on a black rock. It is also known simply as Afyon. Older spellings include Karahisar-i Sahip, Afium-Kara-hissar and Afyon Karahisar. The city was known as Afyon (opium), until the name was changed to Afyonkarahisar by the Turkish Parliament in 2004.

The top of the rock in Afyon has been fortified for a long time. It was known to the Hittites as Hapanuwa, and was later occupied by Phrygians, Lydians and Achaemenid Persians until it was conquered by Alexander the Great. After the death of Alexander the city (now known as Akroinοn (Ακροϊνόν) or Nikopolis (Νικόπολις) in Ancient Greek), was ruled by the Seleucids and the kings of Pergamon, then Rome and Byzantium.

Akroinοn became an important fortress in the Armeniakon theme due to its strategic location and natural defences and was first mentioned in Byzantine history when it was attacked in 716 and 732 by Arab invaders. The Byzantine emperor Leo III renamed the city Nicopolis (Greek for "city of victory") after his victory over Arab besiegers under Abdallah al-Battal (who would become the famous Turkish literature figure of Battal Gazi) in 740. Since the 10th century it was also a bishopric of Phrygia Salutaris.

After 1071 the town became part of the frontier zone between the Byzantine Empire and the invading Turks. The city was still held by the former in 1112 but was lost to the Sultanate of Rum at some time before 1146 when Manuel I Komnenos won a victory here. The Turks were unable to firmly control the city until around 1210, renaming it to Kara Hissar ("black castle") after the ancient fortress situated upon a volcanic rock 201 meters above the town. Following the dispersal of the Seljuqs the town was occupied by the Sâhib Ata and then the Germiyanids.

The castle was finally 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.

From its situation on the route of the caravans between Smyrna and western Asia on the one hand and places such as Armenia and Georgia on the other, the city became a place of extensive trade. It thrived during the Ottoman Empire as the centre of opium production, with Afyon becoming a wealthy city. From 1867 until 1922, Afyon was part of the Hüdavendigâr vilayet of the Ottoman Empire. In 1902, a fire burning for 32 hours destroyed parts of the city.

During the First World War, British prisoners of war who had been captured at Gallipoli were housed here in an empty Armenian church at the foot of the rock. During the Greco-Turkish War (1919-1922) campaign (part of the Turkish War of Independence) Afyon and the surrounding hills were occupied by Greek forces. However, it was recovered on 27 August 1922, a key moment in the Turkish counter-attack in the Aegean region. After 1923 Afyon became a part of the Republic of Turkey.