The Seversky Donets (Се́верский Доне́ц) or Siverskyi Donets (), usually simply called the Donets, is a river on the south of the East European Plain. It originates in the Central Russian Upland, north of Belgorod, flows south-east through Ukraine (Kharkiv, Donetsk and Luhansk Oblasts) and then again through Russia (Rostov Oblast) to join the river Don, about from the Sea of Azov. The Donets is the fourth-longest river in Ukraine, and the largest in eastern Ukraine, where it is an important source of fresh water. It gives its name to the Donets Basin, known commonly as the Donbas, an important coal-mining and industrial region in Ukraine.
The names Don and its diminutive Donets are derived from Iranic, Sarmatian Dānu "the river". Scytho-Sarmatians inhabited the areas to the north of the Black Sea from 1100 BC into the early medieval times.
In the 2nd century CE Ptolemy knew the river Don, into which the Donets flows, as Tanais, and Western Europeans recognized that the Don had a significant tributary which they called either the small Tanais or Donetz. The Slavic name of Seversky Donets derived from the fact that the river originates from the land of Severians. As the Italian-Polish chronicler Alexander Guagnini (1538–1614) wrote: "There is also another, small Tanais, which originates in the Seversky Principality (for this reason it is called Donets Seversky) and flows into the large Tanais above Azov".
The Donets is the largest river in eastern Ukraine and the largest tributary of the Don. Its total length is and the basin area is . Most of the river's length stretches across Ukraine. The average annual flow is near the source and at the confluence to the Don.
The Donets originates on the Central Russian Upland, near Podolkhi village, Prokhorovka area, north of Belgorod, at an elevation of above sea level. Its basin contains over 3000 rivers, of which 425 are longer than and 11 are longer than ; 1011 of those rivers directly flow into the Donets. These rivers are mostly fed by melting snow, and thus the water supply is uneven during the year.