Tigris River, a river in south-western Asia, mainly in Iraq. It begins as the Dicle River in the Kurdistan region of eastern Turkey and flows southeastward along part of the Turkish-Syrian border and then into Iraq. The river joins the Euphrates about 40 miles (64 km) northwest of Basra to form the Shatt al Arab, which continues to the head of the Persian Gulf. The Tigris is about 1,150 miles (1,850 km) long. It is shorter than the Euphrates River, but is swifter, carries more water, and is commercially more important. The largest cities on its banks are Mosul and Baghdad, Iraq.

The river's main tributaries are the Great Zab, Little Zab, Diyala, and Karkheh rivers. Several barrages, or dams, have been built on the Tigris and its tributaries for purposes of flood control and irrigation. The river is navigable by river boats to Baghdad and in the high-water season by small craft to Mosul.

The Tigris forms the eastern boundary of Mesopotamia, and in ancient times the eastern arm of the Fertile Crescent lay on both sides of the river. On the Tigris opposite Mosul are the ruins of Nineveh, the last capital of the Assyrian Empire. Many Biblical events occurred in the region; in the Old Testament the river is sometimes called the Hiddekel.