The Tisza or Tisa is one of the main rivers of Central Europe. It rises in Ukraine, and is formed near Rakhiv by the junction of headwaters White Tisa, whose source is in the Chornohora mountains and Black Tisa, which springs in the Gorgany range. It flows roughly along the Romanian border and enters Hungary at Tiszabecs; downstream, it marks the Slovak-Hungarian border, passes through Hungary, and falls into the Danube in north Serbia (Vojvodina). There, it forms the boundary between the regions of Bačka and Banat. The river also forms short portions of the border between Hungary and Ukraine and between Hungary and Serbia.

The Tisza drains an area of about 156,087 km2 (60,266 sq mi) and has a length of 965 km (600 mi) (the 16th longest river in Europe)—the biggest catchment and length of any of Danube tributaries. With the mean annual discharge of 792 m3/s (28,000 cu ft/s), its contribution to the Danube's total runoff is about 13%.

Names for the river in the countries it flows through are:

• Romanian: Tisa;
• Ukrainian: Тиса (Tysa);
• Slovak: Tisa;
• Hungarian: Tisza;
• Serbian: Тиса, Tisa.

The river was known as the Tisia in antiquity, and Latin names for it included Tissus, Tisia, Pathissus. In the Serbian- and Slovak languages, it is called Tisa. It may be referred to as the Theiss (German: Theiß) in older English references, after the German name for the river.

Attila the Hun is said to have been buried under a diverted section of the river Tisza.

The length of the Tisza in Hungary used to be 1419 km. It flowed through the Great Hungarian Plain, which is one of the largest flat areas in central Europe. Since plains can cause a river to flow very slowly, the Tisza used to follow a path with many curves and turns, which led to many large floods in the area.

After several small-scale attempts, István Széchenyi organised the "control of the Tisza" (Hungarian: a Tisza szabályozása) which started on August 27, 1846 and substantially ended in 1880. The new length of the river in Hungary was 966 km, with 589 km of "dead channels" and 136 km of new riverbed.

The resultant length of the flood-protected river comprises 2,940 km (out of 4,220 km of all Hungarian protected rivers) which forms one of the largest flood protection systems in Europe; larger than the Netherlands' 1,500 km, the Po River's 1,400 km, or the Loire Valley's 480 km.

In the 1970s the building of the Kisköre Reservoir started with the purpose of helping to control floods as well as storing water for drought seasons. It turned out, however, that the resulting Lake Tisza became one of the most popular tourist destinations in Hungary, since it had similar features to Lake Balaton at drastically cheaper prices and it was not crowded.

The Tisza is navigable over much of its course. The river opened up for international navigation only recently; before, Hungary distinguished "national rivers" and "international rivers", indicating whether non-Hungarian vessels were allowed or not. After Hungary joined the European Union, this distinction was lifted and vessels were allowed on the Tisza.

Conditions of navigation differ with the circumstances: when the river is in flood, it is often unnavigable, just as it is at times of extreme drought.

In early 2000, there was a sequence of serious pollution incidents originating from accidental industrial discharges in Romania. The first, in January 2000, occurred when there was a release of sludge containing cyanide from a Romanian mine and killed 2000 tons of fish. The second, from a mine pond at Baia Borsa, northern Romania, resulted in the release of 20,000 cubic metres of sludge containing zinc, lead and copper occurred in early March 2000. A week later, the third spill occurred at the same mining site at Baia Borsa, staining the river black, possibly including heavy metals.

This series of incidents were described at the time as the most serious environmental disaster to hit eastern Europe since the Chernobyl disaster. Use of river water for any purpose was temporarily banned and the Hungarian government pressed the Romanians and the European Union to close all installations that could lead to further pollution.

Examination of river sediments indicates that pollution incidents from mines have occurred for over a century

The following rivers are the major tributaries to the river Tisza: Bodrog, Körös, Mureş and Someş