Brčko (pronounced [br̩̂tʃkoː]) is a city and the administrative seat of Brčko District, in northern Bosnia and Herzegovina. It lies on the banks of Sava river across from Croatia. As of 2013, it has a population of 39,893 inhabitants.

De jure, the Brčko District belongs to both entities of Bosnia and Herzegovina (the Federation of Bosnia and Herzegovina and Republika Srpska) but in practice it is not governed by either; practically, Brčko is a self-governing free city.

Its name is very likely linked to the Breuci (Greek Βρεῦκοι), one of the Pannonian tribes of the Illyrians who migrated to the vicinity of today's Brčko from the territories of the Yamnaya culture in the 3rd millennium BC. Breuci greatly resisted the Romans but were conquered in 1st century BC and many were sold as slaves after their defeat. They started receiving Roman citizenship during Trajan's rule.

A number of Breuci migrated and settled in Dacia, where a town called Bereck or Brețcu, a river (Brețcu River) and a mountain Munții Brețcului in today's Romania were named after them.

The city is on the north, riparian border of Bosnia, across the Sava River from the village of Gunja in Croatia.

Brčko is the seat of the Brčko District, an independent unit of local self-government created on the territory of Bosnia and Herzegovina following an arbitration process. The local administration was formerly supervised by an international supervisory regime headed by Principal Deputy High Representative who is also ex officio the Brčko International Supervisor. This international supervision was frozen since 23 May 2012.

Upon the conclusion of the Treaty of Passarowitz in 1718, Brčko was to be transferred to the Habsburg monarchy.

During the 1862 exodus of Muslims from Serbia, some Belgrade Muslims who were expelled by the Serbian government settled in the town.