A sakellarios (Greek: σακελλάριος) or sacellarius is an official entrusted with administrative and financial duties (cf. sakellē or sakellion, "purse, treasury"). The title was used in the Byzantine Empire with varying functions, and remains in use in the Eastern Orthodox Church.
The first known sakellarios was a certain Paul, a freedman appointed by Emperor Zeno (reigned 474–491). The sakellarios is hence usually assumed to have headed a sakellion (or sakella, sakelle), a term which appears in early Byzantine sources with the apparent sense of "treasury", more specifically of cash, as opposed to the vestiarion which was for goods. Despite the origin of the term, the sakellarioi of the early Byzantine period (5th–7th centuries) are not directly associated with financial matters. Rather they appear connected with the imperial bedchamber (koiton), bearing court titles such as spatharios or koubikoularios, while some holders of the office were entrusted with distinctly non-financial tasks: Emperor Heraclius (r. 610–641) appointed the sakellarios Theodore Trithyrius to command against the Arabs, while another sakellarios conducted the examination of Maximos the Confessor under Constans II (r. 641–668).
It is only in the early 8th century that sakellarioi are directly mentioned as treasurers. By the time of the Taktikon Uspensky of ca. 843, the sakellarios had become a general comptroller of the fiscal bureaux (the sekreta), with notaries reporting to him in each department. The actual head of the sakellion department from this period on became the chartoularios tou sakelliou.
The post continues in evidence until at least 1196, although it may have for a time been subsumed into that of the megas logariastes under Alexios I Komnenos (r. 1081–1118). From the late 11th century, the prefix megas ("grand") was added to it.
Imitating the practice of the imperial court, the Patriarchate of Constantinople had its own sakellion. Like his secular counterpart, the patriarchal sakellarios lost its function as treasurer by the late 11th century and took over the supervision of donations to and the administration of the monasteries of Constantinople. At the same time, it also acquired the prefix megas and replaced the megas skeuophylax as the second-most important official of the patriarchate. By the 13th century, the institution of megas sakellarios had been replicated in the provincial sees as well.