The Wieprz ([ˈvjɛpʂ], boar) is a river in central-eastern Poland, a tributary of the Vistula. It is the country's ninth longest river, with a total length of 303 km and a catchment area of 10,415 km², all within Poland. Its course near the town of Łęczna includes the protected area known as Wieprz Landscape Park.
The river has its sources in Lake Wieprz, near Tomaszow Lubelski, and flows into the Vistula near Deblin. The Wieprz is connected to another river, the Krzna, through the 140-kilometer Wieprz-Krzna Canal, built in 1954 - 1961. Because the Wieprz with its wide valley has not been regulated, its nature is very diverse. The meandering river with its oxbow lakes is inhabited by numerous birds, European otters and Eurasian beavers.
During the Polish-Soviet War, units of the Polish 4th Army concentrated along the Wieprz, getting ready for the Battle of Warsaw. In September 1939, during the Invasion of Poland, the Battle of Tomaszow Lubelski took place by the Wieprz.