The Karun (Persian: کارون, IPA: [kɒːˈɾuːn]), the Ancient Greek Eulaeus (Greek: Εὔλαιος or Εὐλαῖος, Hebrew Ulai (Hebrew: אולי), is the Iranian river with the highest water flow, and the country's only navigable river. It is 950 km (590 mi) long. The Karun rises in the Zard Kuh mountains of the Bakhtiari district in the Zagros Range, receiving many tributaries, such as the Dez and the Kuhrang. It passes through the city of Ahvaz, the capital of the Khuzestan Province of Iran, before emptying to its mouth into Arvand Rud.

The Karun continues toward the Persian Gulf, forking into two primary branches on its delta – the Bahmanshir and the Haffar – that join the Arvand Rud, emptying into the Persian Gulf. The important Island of Abadan is located between these two branches of the Karun. The port city of Khorramshahr is divided from the Island of Abadan by the Haffar branch.

Juris Zarins and other scholars have identified the Karun as one of the four rivers of Eden (Gihon), the others being the Tigris, the Euphrates, and either the Wadi al-Batin or the Karkheh.

In early classical times, the Karun was known as the Pasitigris. The modern medieval and modern name, Karun, is a corruption of the name Kuhrang, which is still maintained by one of the two primary tributaries of the Karun. J. G. Lorimer also records in his Gazetteer of the Persian Gulf, Oman and Central Arabia that it was known by the name "Dujail," which could be translated "Little Tigris," to medieval Arab and Persian geographers.

It originates in the Zagros Mountains of western Iran, on the slopes of 4,221 m (13,848 ft) Zard-Kuh. The river flows south and west through several prominent mountain ridges and receives additional water from the Vanak on the south bank and the Bazoft on the north. These tributaries add to the catchment of the river above the Karun-4 Dam. 25 kilometres (16 mi) downstream, the Karun widens into the reservoir formed by the Karun-3 Dam.

The Khersan flows into a reservoir from the southeast passin through it in a narrow canyon, now in a northwest direction, past Izeh, eventually winding into the Sussan Plain. The Karun then turns north into the reservoir of Shahid Abbaspour Dam (Karun-1), which floods the river's defile to the southwest. The Karun flows southwest into the impoundment of Masjed Soleyman Dam (Karun-2), then turns northwest. Finally, it leaves the foothills and flows south past Shushtar and its confluence with the Dez. It then bends southwest, bisecting the city of Ahvaz, and south through farmland to its mouth on the Arvand Roud at Khorramshahr, where its water, together with that of the Tigris and Euphrates, turns sharply southeast to flow to the Persian Gulf.

The largest river by discharge in Iran, the Karun River's watershed covers 65,230 square kilometres (25,190 sq mi) in parts of two Iranian provinces. The river is around 950 kilometres (590 mi) long and has an average discharge of 575 cubic metres per second (20,300 cu ft/s). The largest city on the river is Ahvaz, with over 1.3 million inhabitants. Other important cities include Shushtar, Khorramshahr (a port), Masjed-Soleyman, and Izeh.

Much of Khuzestan's transport and resources are connected in one way or another to the Karun. Since the British first discovered oil at Masjed Soleyman, the Karun has been an important route for the transport of petroleum to the Persian Gulf, and remains an important commercial waterway. Water from the Karun provides irrigation to over 280,000 hectares (690,000 acres) of the surrounding plain and a further 100,000 hectares (250,000 acres) are planned to receive water.