Tigris (Arabic Dijlah, Turkish Dicle), river in southwestern Asia, rising in Turkey and flowing through Iraq before joining the Euphrates to form the Shatt al Arab, which empties into the Persian Gulf.

The Tigris is 1,900 km (1,180 mi) long and drains an area of more than 110,000 sq km (43,000 sq mi). The river originates in the mountains of eastern Turkey and flows southeast into Iraq after briefly forming the extreme eastern portion of the border between Syria and Turkey. Once in Iraq, the Tigris zigzags slowly to the southeast, and its valley flattens and widens. In southern Iraq the Tigris joins with the Euphrates to form the Shatt al Arab, which is 170 km (110 mi) long and flows to the head of the Persian Gulf. In ancient times the area between the Tigris and the Euphrates was the site of some of the world’s earliest urban civilizations (see Mesopotamia).

Major tributaries of the Tigris are the Great Zab, Little Zab, Diyālá, and Al ‘Uzaym. All of these rivers join the Tigris in Iraq. However, the waters of the Great Zab rise in Turkey and those of the Little Zab and the Diyālá rise in Iran. Major cities on the river include Diyarbakır in Turkey and Mosul and Baghdād in Iraq. The Tigris is too shallow for navigation by all but very small boats, especially south of Baghdād where the river fans out and meanders through dense marshland.

In the past, significant flooding often occurred when melting winter snow in the highlands, together with rains in late winter and early spring, swelled the Tigris. Construction of the Sāmarrā’ Dam in the 1950s enabled the diversion of Tigris waters to the Tharthar depression in central Iraq to assist in flood control. Another measure sought to reduce the significant quantities of silt that the Tigris was carrying into the Shatt al Arab. The excess silt was contributing to the extension of the delta into the Persian Gulf. Flood control projects, as well as the construction of dams on the major tributaries of the Tigris for irrigation, reduced the silt deposits but also decreased the flow of fresh water to southern Iraq. This decreased flow allowed salt water from the Persian Gulf to mix into the lower Tigris. The reduction in fresh water combined with increased pollution to rob the soil of nutrients, harming local agriculture.

The ruins of Nineveh, capital of ancient Assyria, lie on the banks of the Tigris. Among other ruins on the river are those of Seleucia and Ctesiphon.