The Old calendarists are those Eastern Orthodox Churches that continue to use the Julian calendar, namely the Russian and Serbian Orthodox Churches and some others. The term is used to distinguish them from the New calendarists who adopted the Revised Julian calendar.

The term is also used for minorities within the New calendarist churches that refused adopting the new calendar, which they perceived as a masonic and ecumenist project which implied the superiority of the West. They preferred setting up a separate church hierarchy to accepting an unwanted calendar. There are a number of Old calendarist churches, not in communion with each other, active until today mainly among the Greek-speaking churches.