The Byzantine army of the Komnenian era or Komnenian army was a force established by Byzantine emperor Alexios I Komnenos during the late 11th/early 12th century. It was further developed during the 12th century by his successors John II Komnenos and Manuel I Komnenos. From necessity, following extensive territorial loss and a near disastrous defeat by the Normans of southern Italy at Dyrrachion in 1081, Alexios constructed a new army from the ground up. This new army was significantly different from previous forms of the Byzantine army, especially in the methods used for the recruitment and maintenance of soldiers. The army was characterised by an increased reliance on the military capabilities of the immediate imperial household, the relatives of the ruling dynasty and the provincial Byzantine aristocracy. Another distinctive element of the new army was an expansion of the employment of foreign mercenary troops and their organisation into more permanent units. However, continuity in equipment, unit organisation, tactics and strategy from earlier times is evident. The Komnenian army was instrumental in creating the territorial integrity and stability that allowed the Komnenian restoration of the Byzantine Empire. It was deployed in the Balkans, Italy, Hungary, Russia, Anatolia, Syria, the Holy Land and Egypt.

Alexios I Komnenos, Latinized Alexius I Comnenus, was Byzantine emperor from 1081 to 1118. Although he was not the first emperor of the Komnenian dynasty, it was during his reign that the Komnenos family came to full power and initiated a hereditary succession to the throne. Inheriting a collapsing empire and faced with constant warfare during his reign against both the Seljuq Turks in Asia Minor and the Normans in the western Balkans, Alexios was able to curb the Byzantine decline and begin the military, financial, and territorial recovery known as the Komnenian restoration. His appeals to Western Europe for help against the Turks was the catalyst that sparked the First Crusade.