Zab (Turkish: Zap suyu, Kurdish: زێ, Persian: زاب; Zâb, Syriac: ܙܒܐ; Zawa) is the name given to two separate rivers that flow through Iran, Iraq and Turkey to become the two principal tributaries of the Tigris. The two rivers, named Greater Zab and Little Zab, were the basis of the ancient Assyrian civilisation. The name Zab is from the Persian word zehâb (زهاب), meaning "water flowing from the ground".

The rivers are extensively used for irrigation and hydroelectricity, with major dams on both rivers. During the spring, the Great and Little Zab are in flood, and, together, double the flow of the Tigris. The ancient city of Assur sits on the west bank of the Tigris, about midway between the respective, east-bank confluences of the two Zabs with the Tigris.

Great Zab

The Great, or Upper, Zab (Kurdish: زێ گه‌وره‌ , Zê Gewre ,Persian زاب بزرگ; Zâb-e Bozorg, Syriac: ܙܒܐ ܥܠܝܐ; Zawa `elaya, Turkish: Büyükzap Suyu, ancient Greek: Λύκος, Lykos; Latin: Lycus) rises in the mountains of southeastern Turkey and flows south for 426 km (265 miles) into Iraq before joining the Tigris south of the city of Mosul at ancient Calah. It forms the approximate boundary of the Kurdish-populated region of Iraq and is used as the political boundary of the Kurdish Autonomous Region. In 750 CE, the Great Zab was the scene of the Battle of the Zab between the Umayyads and the Abbasids. The yet unfinished Bakhma Dam on the Great Zab, near Shaqlawa, Arbil Governorate, could control 14.4 km³ of water for hydroelectricity and irrigation. Work on the dam began in the late 1980s, but halted in 1991 due to the Gulf War and the economic sanctions imposed on Iraq by the United Nations afterwards.

Before the Assyrian Genocide during World War I, the Great Zab was a major river for the Assyrian (also known as Chaldean and Syriac, among other names) population in the Hakkari region in modern-day southeastern Turkey. Because of this, as well as its ancient importance as a basis for the Assyrian civilisation, the Great Zab is represented on the Assyrian flag by four white streams flowing from the flag's centre.

Little Zab

The Little, or Lower, Zab (الزاب الاسفل: al-Zāb al-Asfal, Persian: زاب کوچک; Zâb-e Kuchak, Syriac: ܙܒܐ ܬܚܬܝܐ; Zawa takhtaya; ancient Greek: Καπρος, Kapros; Latin: Caprus) rises in northwestern Iran, in the north of Piranshahr city and flows southwest for 402 km (250 miles) through Iraq to join the Tigris north of the town of Baiji. The Dokan Dam in Iraqi Kurdistan is built on the Little Zab.