Doboj (Serbian Cyrillic: Добој, pronounced [dôboj]) is a city in Republika Srpska, Bosnia and Herzegovina. It is situated on the banks of the Bosna river, in the northern region of Republika Srpska. As of 2013, it has a population of 71,441 inhabitants.

Doboj is the largest national railway junction and the operational base of the Railways Corporation of Bosnia and Herzegovina. It is one of the oldest cities in Bosnia and Herzegovina and, aside from Banja Luka, the most important urban center in northern Republika Srpska.

Prior to the Bosnian War, the municipality of Doboj had a larger surface area. Most of the pre-war municipality's territory is part of Republika Srpska, including the city itself. The southern rural areas are part of the Zenica-Doboj Canton of the Federation of Bosnia and Herzegovina, and the eastern rural part of the municipality is part of the Tuzla Canton, also in the Federation of Bosnia and Herzegovina. The parts of the pre-war Doboj Municipality that are in the Federation of Bosnia and Herzegovina are the municipalities of Doboj South (Doboj Jug), Doboj East (Doboj Istok) and the Municipality of Usora. The northern suburbs of Doboj extend into the Pannonian plains, and effectively mark the southern tip of this great Central European plain. The southern (Doboj South) and eastern suburbs (Doboj East) are spread on the gentle hills which extend to the larger Central Bosnian mountain areas (Mt. Ozren in the southeast, Mt. Krnjin in the west).

Doboj has been continuously inhabited ever since the Neolithic times. Fragments of pottery and decorative art were found on several localities, with the most known site in Makljenovac, south from the city proper, at the confluence of the Usora and Bosna rivers. Archeological findings from the Paleolithic era were found in a cave in the Vila suburb.

The Illyrian tribe of Daesitates settled in this region as early as the twelfth century BC. Daesitates were one of the largest and most important Illyrian tribes residing at the territory of modern-day Bosnia and Herzegovina, sharing their northern borders with Breuci, another important tribe. Daesitates and Breuci started the Great Illyrian Revolt, or in Roman sources, the widespread rebellion known as Bellum Batonianum (6–9 AD). After the bloody rebellion was subdued, Roman legions permanently settled in the area and built a large military camp (Castrum) and a civilian settlement (Canabea) in Makljenovac. These structures were most likely built in the early Flavian dynasty era, during Vespasian's rule.

The military camp was large, in the shape of a near perfect rectangle with large towers at each corner and the main gate in the middle of the central wall, and served as the most important defense on the old Roman road from Brod to Sarajevo, demarcating the very borders of the Roman provinces of Dalmatia and Pannonia. It served its role for several centuries with the evidence of Belgian and Spanish cohorts stationed there in the second and third century AD. Canabea contained Roman settlers, with evidence of a large bathhouse with a hypocaust (central heating) and a concubine house for soldiers stationed at the nearby Castrum. A large Villa Rustica was located in the modern-day suburb of Doboj, appropriately named Vila. Very fine pieces of religious and practical artefacts were found at these sites, including an altar dedicated to Jupiter, figurines of Mars, and fragments of African made Terra sigillata pottery. When South Slavic tribes migrated into this area in the sixth and seventh century AD, they had settled initially on the ruins of the previous Roman settlement and lived there continuously until the early thirteenth century at which point they used stones and building material from the old Roman Castrum in order to build the stone foundation of the Gradina fortress, several kilometers north, in the modern-day old town of Doboj. Nowadays only the walls of the former camp and civilian settlement are still open to visitors.

The first official mention of the city itself is from 1415, in a charter issued by Dubrovnik to the Holy Roman Emperor Sigismund, although there are numerous artefacts and objects that have been found (kept in the National Museum of Bosnia and Herzegovina in Sarajevo and the Regional Museum in Doboj), which confirm that the area had been inhabited ever since the early Stone Age, and that the Roman Empire had an army camp (Castrum) and a settlement (Canabea) in the vicinity of the town dating from the first century AD. Following the arrival of the Slavs in the sixth century it became a part of the region/Usora banate (in medieval documents sometimes collectively mentioned with the nearby province of Soli, hence, Usora and Soli).

The Doboj fortress, a royal Kotromanić fortress, was first built in the early thirteenth century and then expanded in the early fifteenth century (1415). It was expanded again during Ottoman rule in 1490. This newer stone foundation of the fortress was built on previous layers of an older foundation (dating back to the ninth or tenth century) made of wood, mud and clay (Motte-and-bailey type). It was a very important obstacle for invaders coming from the north, Hungarians, and later on, Austrians and Germans. It was built in the Gotho-Roman style with Gothic towers and Romanesque windows. The area saw numerous battles in medieval times and the fortress often changed hands between Bosnian and Hungarian armies. Doboj was the site of a particularly major battle between the Hungarians and a Bosnian-Turkish coalition in early August 1415 in which the Hungarians were heavily defeated on the field where the modern city of Doboj lies today. As an important border fortress between the Bosnian Kingdom and Hungary it was also frequently attacked, officially recorded as 18 times, in the Austro-Ottoman Wars, and fell to the Austro-Hungarians in 1878.