The Tisza, Tysa or Tisa, is one of the major rivers of Central and Eastern Europe. It was once called "the most Hungarian river" because it used to flow entirely within the Kingdom of Hungary. Today, it crosses several national borders.

The Tisza begins near Rakhiv in Ukraine, at the confluence of the White Tisa [uk] and Black Tisa [uk], which is at coordinates 48°4′29″N 24°14′40″E (the former springs in the Chornohora mountains; the latter in the Gorgany range). From there, the Tisza flows west, roughly following Ukraine's borders with Romania and Hungary, then briefly as the border between Slovakia and Hungary, before entering into Hungary, and finally into Serbia. The Tisza enters Hungary at Tiszabecs, traversing the country from north to south. A few kilometers south of the Hungarian city of Szeged, it enters Serbia. Finally, it joins the Danube near the village of Stari Slankamen in Vojvodina, Serbia.

The Tisza drains an area of about 156,087 km² (60,266 sq mi)^[2] and has a length of 966 km (600 mi)^[12] Its mean annual discharge is seasonally 792 m³/s (28,000 cu ft/s) to 1,050 m³/s (37,000 cu ft/s). It contributes about 13% of the Danube's total runoff.^[2]

Attila the Hun is said to have been buried under a diverted section of the river Tisza.^[13]

The river was known as the Tisia in antiquity; other ancient names for it included Pathissus (Πάθισσος in Ancient Greek and later Tissus (in Latin)), (Pliny, Naturalis historia, 4.25). It may be referred to as the Theiss in older English references, after the German name for the river, Theiß. It is known as the Tibisco in Italian, and in older French references (as for instance in relation to the naval battles on the Danube between the Ottoman Empire and the Habsburg Empire in the 17th and 18th centuries) it is often referred to as the Tibisque.

Modern names for the Tisza in the languages of the countries it flows through include:

• Romanian: Tisa, pronounced [ˈtisa];
• Ukrainian: Тиса, romanized: Tysa, pronounced [ˈtɪsɐ];
• Slovak: Tisa, pronounced [ˈtisa];
• Hungarian: Tisza, pronounced [ˈtisɒ];
• Serbo-Croatian Cyrillic: Тиса / Serbo-Croatian Latin: Tisa, pronounced [tîsa].

The length of the Tisza in Hungary used to be 1,419 km (882 mi). 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 "regulation 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 reduced to 966 km (600 mi) in total, with 589 km (366 mi) of dead channels and 136 km (85 mi) of new riverbed.^[14]

In the 1970s, the building of the Tisza Dam at Kisköre started with the purpose of helping to control floods as well as storing water for drought seasons. However, 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 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.^[15]

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.^[16]

The Tisza has a rich and varied wildlife. Over 200 species of birds reside in the bird reserve of Tiszafüred. The flood plains along the river boast large amounts of diverse plant and animal life. In particular, the yearly "flowering" of the Tisza is considered a local natural wonder. The flowering attracts vast numbers of mayflies which is a well known spectacle.^[17][18]

In September 2020, colonies of magnificent bryozoans were discovered in the river.^[19]

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 m³ (710,000 cu ft) 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.^[20]

This series of incidents were described at the time as the most serious environmental disaster to hit central Europe since the Chernobyl disaster. Usage 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.^[20]

Examination of river sediments indicates that pollution incidents from mines have occurred for over a century.^[21]

• Tice (wetlands)
• Ečka fish pond