The collapse of the Roman Empire was followed by a series of invasions by barbarian tribes, such as the Ostrogoths, Gepidaes, Sarmatians, Avars, and Slavs. As a consequence of its exposed position on the border - the limes - Belgrade was often attacked and destroyed. Attacks directed from the north across the Pannonian Depression and the Danube and the Sava were so fierce that not even Singidunum, as a major military fortress, was able to withstand them. Consequently, the Huns were able to conquer and completely destroy the town in 441. This is when the town lost its former Roman population. After the Huns fell, the town once again (454) became part of the Byzantine Empire, but was soon conquered by the Sarmatians and then by the Ostrogoths. However, by 488, the town had yet again returned to Byzantine control.

At the beginning of the sixth century (512), Byzantine Emperor Anastasius allowed the Heruli, a German tribe, to settle in the immediate vicinity of the town in order to protect it from the aggressive Gepidaes. Traces of a Germanic material culture have been found among the ruins of the former Roman town, in the western side of the Lower Town. During the rule of Byzantine Emperor Justinian I, the town was rebuilt and surrounded by strong walls in 535 and thus Singidunum reclaimed its former glory and became a town of great renown. Probably for the first time, the town was connected with Taurunum on the opposite river bank.

At the end of the sixth century, while the Byzantines were preoccupied with wars in Africa and Asia, the Mongol Avars reached the town, closely followed by the first Slavs. They launched two sieges against the town, which resulted in the Avars and Slavs conquering Singidunum. Following this barbarian onslaught and the subsequent destruction of the town, the name Singidunum was never again used to refer to Belgrade. Traces of a Slavic material culture dating from this period have been found in the Lower and Upper Towns, in Zemun, Ritopek and Višnjica. This period marks the permanent Slavicisation of Belgrade.

Around 630, the Serbs migrated to the area and since the town was no longer a border fortress, the Avars and Slavs were not as concerned with it, and for two and a half centuries there are no historical records. At that time the town was located within the wider area of the Balkan Peninsula, which had already been conquered. In spite of this, archaeological findings point to evidence of continuous settlement of the town and its surroundings. The next time the town is mentioned is in the ninth century, 16^th April, 878 to be exact, in a letter dated from Pope John VIII to Bulgarian Khan Boris Mihailo about the dismissal of Sergije, a Christian bishop in Belgrade. This time, however, the Slavic word Beograd ('White City' - most probably because of the white limestone walls) is used to refer to the town. Following on from this, the name appears in a variety of different languages: Alba Graeca, Griechisch Weissenburg, Nandor Alba, Nandor Fejervar, Castelbianco, Alba Bulgarica.

During the next few centuries, after the first mention of Belgrade as a Slavic town, it was passed between various armies and occupying forces. The Franks were the first to reach Belgrade and led by Charles the Great they destroyed the Avars. They established a Frankish settlement called Mallevila among the ruins of Taurunum, which was later referred to by the Slavic name Zemln (Zemun). The Bulgarians came next and replaced the Franks as the town's rulers, and then they themselves were supplanted by the Hungarians. At the end of the tenth century, Belgrade changed hands yet again when it became part of Samuel's great empire. In 1018, Belgrade once again became a significant border fortress within the Byzantine Empire. During the eleventh and twelfth centuries, the rival forces of Hungary, the Byzantine Empire and Bulgaria fought over Belgrade.

It was during this period of history that numerous Christian Crusades passed through as they journeyed to the East, wreaking destruction on Belgrade on the way. Following the Crusades of 1096 and 1147, Frederick Barbarossa led 190,000 crusaders through Belgrade in 1189. Just how much the city suffered during this Crusade can be clearly pictured by looking at the description of Belgrade made by Arabian geographer and cartographer al-Idrisi, in his 1154 work, "Itinerary of the Byzantine Road", when he described "Belgraduk" as a populous and lively town, with many churches.