Old Believers or Old Ritualists (Russian: староверы, starovery or старообрядцы, staroobryadtsy) is the common term for several religious groups, which maintain the old liturgical and ritual practices of the Russian Orthodox Church, as they were before the reforms of Patriarch Nikon of Moscow between 1653 and 1657. The old rite and its followers were anathematized in 1667, and Old Belief gradually emerged from the resulting schism.

The antecedents of the movement regarded the reform as heralding the End of Days, and the Russian church and state as servants of the Antichrist. Fleeing persecution by the government, they settled in remote areas or escaped to the neighboring countries. Their communities were marked by strict morals and religious devotion, including various taboos meant to separate them from the outer world. They rejected the Westernization measures of Peter the Great, preserving traditional Russian culture, like long beards for men.

Lacking a central organization, the main division within Old Belief is between the relatively conservative popovtsy, or "priestly", who were willing to employ renegade priests from the state church, maintaining the liturgy and sacraments; and the more radical bezpopovtsy, or "priestless", who rejected the validity of "Nikonite" ordination, and had to dispense with priests and all sacraments performed by them, appointing lay leaders instead. Various polemics produced numerous subdivisions, known as "accords". Old Belief covers a spectrum ranging from the established and hierarchic "priestly" Russian Orthodox Old-Rite Church, to the anarchistic "priestless" Fugitives.

From the mid-18th century, under Catherine the Great, Old Believers gained nearly complete tolerance, and large urban centers emerged, the members of which had a leading role in Russian economy and society. Persecution and discrimination were renewed under Alexander I, and especially Nicholas I, from 1820 onward. Total freedom of religion and equal rights were granted by the Russian Revolution of 1905, followed by a brief golden age. In the beginning of the 20th century, demographers estimated the number of Old Believers to have been between 10 million and 20 million. The destruction wrought during the Stalin era decimated the communities, leaving few who adhered to their traditions, and a wave of refugees established new centers in the West. The movement enjoys a renewal in the post-Soviet states, and in the dawn of the 21st century, there are over 1 million Old Believers who reside mostly in Russia, Latvia, Lithuania, Romania, Ukraine, Belarus, Estonia, and the United States.

While Old Belief is highly diverse, all its branches are defined above all else by the rejection of the liturgical and ritual reforms enacted in the Russian Orthodox Church between 1653 and 1657, and by strict adherence to the Russian rite and traditions which preceded them. Instituted by Patriarch Nikon, the reforms were intended to eliminate all differences between the Russian use and that of the Greek Orthodox Churches: wherever a certain detail in local custom was found to diverge, it was corrected to resemble the parallel Greek one. The reform was not concerned with theology, and in this respect, there is no real difference between the Old Believers and the official Orthodox Church. It did touch upon numerous matters of form, totaling hundreds of pages in details.

Some of these changes are discernible and easily distinguish Old Believers from the "Nikonian" rite, as they term it. The best known, which became a symbol of contention, is the manner of crossing oneself: pre-reform Russian custom, retained by Old Believers, is to fold together the thumb, ring and little fingers, while holding the index and middle fingers upright, known as "crossing with two fingers"; the "new" rite is to fold the thumb, index and middle fingers together, in "three fingers". Old Believers recognize only baptism by triple full immersion, and eschew baptism by pouring, which is acceptable in the new rite; the symbol of the cross is always the eight-pointed Orthodox cross, not any other variant; the Alleluia after the psalmody is recited twice, not thrice; and during the Divine Liturgy, seven prosphora are served rather than five. The procession around the church is directed clockwise, not counter-clockwise. Old Believers perform numerous bows and prostrations, using a prayer mat called podruchik, mostly abandoned in the new rite.

Old Believers spell the name of Christ in Russian with a single I and not two, as Isus and not Iisus. The phrase "ages of ages" is rendered in the dative, veki vekom, and not in the genitive veki vekov, as in the new rite. In the Nicene Creed, the title "True", istinnago, precedes the words "Lord and Giver of Life", and the Kingdom "has no" (nest') rather than "shall have no (ne budet) end". Apart from those, there are countless liturgical and ritual differences, including the names of the saints and rulers mentioned during the Liturgy of Preparation, the wording of the Ektenia for the dead, and so forth.

Breaking with the official Church over the reform, the movement ignores all the innovations and decisions of Russian Orthodoxy since the mid-17th century. New saints canonized since, like Seraphim of Sarov, are not venerated by Old Believers, who have adopted new saints of their own, like Archpriest Avvakum. In the field of religious music, Old Believers retain the monophonic, unison Znamenny chant, which has its own distinct notation style, and do not employ the Part song imported to Russia from the Greek churches. In the field of icon painting, Old Believer artists carefully preserved the otherworldly style of the medieval Orthodox icon and eschewed Western-influenced realistic perspective or natural colours. Animalistic representations of certain saints or certain styles of depicting Jesus, banned by the established church, continued to appear in the movement's icons. Old Believer clerical vestments do not include items of clothing that became fashionable since Nikon's time, like the Greek klobuk and kamilavka.