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.

At the beginning of the Komnenian period in 1081, the Byzantine Empire had been reduced to the smallest territorial extent in its history. Surrounded by enemies, and financially ruined by a long period of civil war, the empire's prospects had looked grim. The state lay defenceless before internal and external threats, as the Byzantine army had been reduced to a shadow of its former self. During the 11th century, decades of peace and neglect had reduced the old thematic forces, and the military and political anarchy following the Battle of Manzikert in 1071 had destroyed the professional Imperial Tagmata, the core of the Byzantine army. At Manzikert, units tracing their lineage for centuries back to the Roman Empire were wiped out, and the subsequent loss of Anatolia deprived the Empire of its main recruiting ground. In the Balkans, at the same time, the Empire was exposed to invasions by the Norman Kingdom of Sicily, the expansionist activities of the principality of Dioclea (Duklja) and by Pecheneg (Patzinak) raids across the Danube.

The death knell of the traditional Byzantine army was at the Battle of Dyrrachion in 1081, where Alexios I was very heavily defeated by the Normans of southern Italy. The nadir of the Byzantine army as a professional fighting force was reached in 1091, when Alexios managed to field only 500 soldiers from the Empire's regular soldiery. These formed the nucleus of the army, with the addition of the armed retainers of Alexios' relatives and the nobles enrolled in the army, plus the substantial aid of a large force of allied Cumans, which won the Battle of Levounion against the Pechenegs. Yet, through a combination of improved finances, skill, determination, and years of campaigning, Alexios, John, and Manuel Komnenos managed to restore the power of the Byzantine Empire, constructing a new army in the process. These developments should not, however, at least in their earlier phases, be seen as a planned exercise in military restructuring. In particular, Alexios I was often reduced to reacting to events rather than controlling them; the changes he made to the Byzantine army were largely done out of immediate necessity and were pragmatic in nature.

The new force had a core of units which were both professional and disciplined. It contained guards units such as the Varangians, the vestiaritai, the vardariotai and also the archontopouloi (the latter recruited by Alexios from the sons of dead Byzantine officers), foreign mercenary regiments, and also units of professional soldiers recruited from the provinces. These provincial troops included kataphraktoi cavalry from Macedonia, Thessaly and Thrace, plus various other provincial forces. Alongside troops raised and paid for directly by the state the Komnenian army included the armed followers of members of the wider imperial family, its extensive connections, and the provincial aristocracy (dynatoi). In this can be seen the beginnings of the feudalisation of the Byzantine military.

The Komnenian period, despite almost constant warfare, is notable for the lack of military treatise writing, which seems to have petered out during the 11th century. So, unlike in earlier periods, there are no detailed descriptions of Byzantine tactics and military equipment. Information on military matters in the Komnenian era must be gleaned from passing comments in contemporary historical and biographical literature, court panegyrics and from pictorial evidence.

There are no surviving reliable and detailed records to allow the accurate estimation of the overall size of the Byzantine army in this period; it is notable that John Birkenmeier, the author of the definitive study of the Komnenian army, made no attempt to do so. He merely noted that while Alexios I had difficulty raising sufficient troops to repel the Italo-Normans, John II could field armies as large as those of the Kingdom of Hungary and Manuel I assembled an army capable of defeating the large crusading force of Conrad III.

Other historians have, however, made attempts to estimate overall army size. During the reign of Alexios I, the field army may have numbered around 20,000 men. By 1143, the entire Byzantine army has been estimated to have numbered about 50,000 men and continued to remain about this size until the end of Manuel's reign. The total number of mobile professional and mercenary forces that the emperor could assemble was about 25,000 soldiers while the static garrisons and militias spread around the empire made up the remainder. During this period, the European provinces in the Balkans were able to provide more than 6,000 cavalry in total while the Eastern provinces of Anatolia provided about the same number. This amounted to more than 12,000 cavalry for the entire Empire, not including those from allied contingents.

Modern historians have estimated the size of Komnenian armies on campaign at about 15,000 to 20,000 men, but field armies with less than 10,000 men were quite common. In 1176 Manuel I managed to gather approximately 30,000–35,000 men, of which 25,000 were Byzantines and the rest were allied contingents from Hungary, Serbia, and Antioch, though this was for an exceptional campaign. His military resources stretched to putting another, smaller, army in the field simultaneously. After the death of Manuel I, the Byzantine army seems to have declined in numbers. In 1186, Isaac II assembled 250 knights and 500 infantry from the Latin population of Constantinople, an equivalent number of Georgian and Turkish mercenaries, and about 1,000 Byzantine soldiers. This force of possibly 2,500 managed to defeat Alexios Branas' rebellion. The rebel army which could not have numbered much more than 3,000–4,000 men had been the field force sent against the Bulgarians. Another force of about 3,000–4,000 was stationed at the city of Serres. Expeditionary forces remained around the same size for the rest of Angeloi period. In 1187 Isaac II campaigned with 2,000 cavalry in Bulgaria. Manuel Kamytzes' army that ambushed Emperor Frederick Barbarossa in 1189 was about 3,000 strong; it consisted of his own force of 2,000 cavalry and the garrison at Philippopolis.