The Dnieper or Dnepr (/(də)ˈniːpər/ (də-)NEE-pər), also called Dnipro (/dəˈniːproʊ/ də-NEE-proh), is one of the major transboundary rivers of Europe, rising in the Valdai Hills near Smolensk, Russia, before flowing through Belarus and Ukraine to the Black Sea. Approximately 2,200 km (1,400 mi) long, with a drainage basin of 504,000 square kilometres (195,000 sq mi), it is the longest river of Ukraine and Belarus and the fourth-longest river in Europe, after the Volga, Danube, and Ural rivers.

In antiquity, the river was part of the Amber Road trade routes. During the Ruin in the later 17th century, the area was contested between the Polish–Lithuanian Commonwealth and Russia, dividing what is now Ukraine into areas described by its right and left banks. During the Soviet period, the river became noted for its major hydroelectric dams and large reservoirs. The 1986 Chernobyl disaster occurred on the Pripyat River, a tributary of the Dnieper, just upstream from its confluence with the Dnieper. The Dnieper is an important navigable waterway for the economy of Ukraine and is connected by the Dnieper–Bug Canal to other waterways in Europe. During the 2022 Russian invasion of Ukraine, certain segments of the river were made part of the defensive lines between territory controlled by the Russians and the Ukrainians.

The river is also sometimes called by the Russian name Dnepr (Днепр, pre-revolutionary spelling Днѣпръ, Dněpr).The initial D in Dnieper is generally silent when pronounced in English, although it may be sounded: /ˈniːpər/ or /dəˈniːpər/.

Dnipro derives from Ukrainian: Дніпро, romanized: Dnipro. The English pronunciation is /dəˈniːproʊ/. The Ukrainian name has a rare form Дніпр, Dnipr and rare dialectal Дніпер, Dniper. The Middle Ukrainian form attested in the sixteenth to eighteenth centuries was Днѣпръ, Dnǐpr. The city of Dnipro is named for the river.

In Belarusian, the river is called Дняпро, Dniapro, or Днепр, Dniepr.

These names are all cognate, deriving from Old East Slavic Дънѣпръ (Dŭněprŭ). The origin of this name is disputed but generally derived from either Sarmatian *Dānu Apara ("Farther River") in parallel with the Dniester ("Nearer River") or from Scythian *Dānu Apr ("Deep River") in reference to its lack of fords, from which was also derived the Late Antique name of the river, Δάναπρις Danapris, as found in the Ravenna Cosmography.

The earlier Graeco-Roman name of the river, as attested by Herodotus, was "Borysthenes" (Ancient Greek: Βορυσθένης, romanized: Borusthénēs; Latin: Borysthenes, Ukrainian: Бористен, Борисфен, romanized: Borysten, Borysfen) and later Δάναπρις Danapris. The name Borysthenes was derived from a Scythian name whose form was:

Ovid used Borysthenius, an adjective derived from Borysthenes, as the river's poetic Latin name.