The Kızılırmak (Turkish pronunciation: [kɯzɯlɯrmak], Turkish for "Red River"), once known as the Halys River (Ancient Greek: Ἅλυς) and Alis River, is the longest river flowing entirely within Turkey. It is a source of hydroelectric power and is not used for navigation.

The Kızılırmak flows for a total of 1,355 kilometres (842 mi), rising in Eastern Anatolia around 39°48′N 38°18′E / 39.8°N 38.3°E / 39.8; 38.3 (Kızılırmak source), flowing first to the west and southwest until 38°42′N 34°48′E / 38.7°N 34.8°E / 38.7; 34.8, then forming a wide arch, the "Halys bend", flowing first to the west, then to the northwest, passing to the northeast of Lake Tuz (Tuz Gölü in Turkish), then to the north and northeast, where it is joined by its major tributary, the Delice River (once known in Greek as the Cappadox river) at 40°28′N 34°08′E / 40.47°N 34.14°E / 40.47; 34.14. After zigzagging to the northwest to the confluence with the Devrez River at 41°06′N 34°25′E / 41.10°N 34.42°E / 41.10; 34.42, and back to the northeast, it joins the Gökırmak (Sky River in Turkish) before finally flowing via a wide delta into the Black Sea northwest of Samsun at 41°43′N 35°57′E / 41.72°N 35.95°E / 41.72; 35.95 (Kızılırmak mouth).

The Hittites called the river the Maraššantiya, and it formed the western boundary of Hatti, the core land of the Hittite empire. Until the Roman conquest of Anatolia the Halys River (later renamed the Kızılırmak by the Turkish conquerors) served as a natural political boundary in central Asia Minor, first between the kingdom of Lydia and the Persian Empire, and later between the Pontic Kingdom and the Kingdom of Cappadocia. As the site of the Battle of Halys, or the Battle of the Eclipse, on May 28, 585 BC, the river formed the border between Lydia to the west and Media to the east until Croesus of Lydia crossed it to attack Cyrus the Great in 547 BC. He was defeated and Persia expanded to the Aegean Sea.

In the 1st century AD Vespasian combined several provinces, including Cappadocia, to create one large province with its eastern boundary marked by the Euphrates River. This province once again splintered during Trajan's reign - the newly created province of Cappadocia, bounded by the Euphrates to the East, included Pontus and Lesser Armenia. The Halys River became an interior river and never regained its significance as a political border. In the 130s a governor of Cappadocia wrote: "long ago the Halys River was the boundary between the kingdom of Croesus and the Persian Empire; now it flows under Roman dominion."

The river's water is used to grow rice and in a few areas water buffalo are kept. There are dams on the river at Boyabat, Altınkaya and Derbent. Dams have reduced the flow of sediment to the delta, allowing coastal erosion.

• Livius.org: Halys - Photos
• sdu.dk/halys