Hebdomadarius, or Hebdomadius, a member of a chapter or convent, whose week it is to officiate in the choir, to rehearse the anthems and prayers, and to perform the usual functions which the superiors perform at solemn feasts, and other extraordinary occasions. The word is formed from the Greek ἑβδομάς, which signifies a week, from ἕξ, seven.

The hebdomadary generally collated to the benefices which became vacant during his week, though this was usually looked upon as an abuse. In cathedrals, the hebdomadary was a canon or prebendary, who had the peculiar care of the choir, and the inspection of the offices for his week. In monasteries, the hebdomadary is he who waits at table for a week or other stated period, and directs and assists the cook.