The Bug or Western Bug is a major river in Central Europe that flows through Belarus (border), Poland, and Ukraine, with a total length of 774 kilometres (481 mi). A tributary of the Narew, the Bug forms part of the border between Belarus and Poland for 178 kilometres (111 mi) and part of the border between Ukraine and Poland for 185 kilometres (115 mi).

The Bug is connected with the Dnieper by the Dnieper–Bug Canal. Out of its 38,712 square kilometres (14,947 sq mi) drainage basin, half is in Poland, just over a quarter in Belarus, and slightly under a quarter in Ukraine.

According to Zbigniew Gołąb, the Slavic hydronym Bug as *bugъ/*buga derives from the Proto-Indo-European verbal root *bheug- (with cognates in old Proto-Germanic *bheugh- etc. with the meaning 'bend, turn, move away'), with the hypothetical original meaning 'pertaining to a (river) bend', and derivatives in Russian búga 'low banks of a river overgrown with bushes', Polish bugaj 'bushes or woods in a river valley or on a steep river bank', and Latvian bauga 'marshy place by a river'.

Traditionally (e.g., by the drafters of the Curzon Line), the Bug River has been considered the ethnographic border between the East and West as well as the border between Orthodox (Ukrainians, Belarusians) and Catholic (Poles) peoples, with Podlachia being a historical borderland where ethnic elements of those nations intermerged.^{[verification needed]}

The Bug was part of the frontier between the territories occupied by Austria, Russia, and Prussia after the Third Partition of Poland in 1795, the southern half of the eastern border of the Duchy of Warsaw and Lithuanian Provisional Governing Commission (1809–1815), Congress Poland and Russia proper (1815–1867), of the Vistula Land and Russia proper (1867–1913), and of the Regency Kingdom of Poland and BPR (1917–1918). The Bug also formed part of the dividing line between German Wehrmacht and Soviet Red Army zones specified in a clause of the German–Soviet Frontier Treaty of 28 September 1939 following the September 1939 invasion of Poland in the Second World War.

The Bug is a left tributary of the Narew. It flows from the Lviv Oblast in the west of Ukraine northwards into the Volyn Oblast, before passing along the Ukraine-Polish and Polish-Belarusian border and into Poland, where it follows part of the border between the Masovian and Podlaskie Voivodeships. It joins the Narew at Serock, a few kilometers upstream of the artificial Zegrze Lake, which was constructed in 1963 with a hydroelectric complex.

This part of the Narew between the confluence and the Vistula is sometimes referred to as Bugo-Narew but on December 27, 1962, the Prime Minister of Poland's act abolished the name "Bugo-Narew", soon after Zegrze Lake was completed.

On the Bug, a few kilometers from the Vysokaye in Kamenets District of the Brest Region, is the westernmost point of Belarus. It is also connected with the Dnieper via the Mukhavets, a right-bank tributary, by the Dnieper-Bug Canal.