The tzaousios (Greek: τζαούσιος) was a late Byzantine military office, whose exact functions and role are somewhat unclear.^[1]

The term is derived from the Turkish çavuş, meaning "courier" or "messenger",^[1] and was in use by the Byzantines perhaps as early as the late 11th century.^[2] In the 13th–15th centuries, it became applied to officers serving in provincial posts. A tzaousios could serve as commander of the garrison of a kastron (a fortified administrative center run by a kephale), possibly combining the military and administrative roles, or as an officer to the megala allagia of the imperial field army.^[1][2] Most of the tzaousioi mentioned in the sources came from the Byzantine Morea,^[3] where they played an important role in provincial administration. In Macedonia and Thrace by contrast, they seem to have been limited to a purely military role within the megala allagia.^[2]

The variant megas tzaousios (μέγας τζαούσιος, "grand tzaousios") is a court title first attested under John III Vatatzes (r. 1221–1254). His functions are unclear.^[4] The French Byzantinist Rodolphe Guilland suggested that he was in command of subordinated tzaousioi, who acted as the successors of the earlier imperial courier corps, the mandatores.^[5] In pseudo-Kodinos's mid-14th century Book of Offices, he is described as being responsible for maintaining the order of the imperial retinue.^[3] Certainly, the first megas tzaousios, Constantine Margarites, was the commander of Vatazes's personal retinue,^[6] but in later times, the title does not appear to have corresponded to a specific function.^[7]