Folk religion, traditional religion, or vernacular religion comprises, according to religious studies and folkloristics, various forms and expressions of religion that are distinct from the doctrines and practices of organized religion. The precise definition of folk religion varies among scholars. Sometimes also termed popular belief, it consists of ethnic or regional religious customs under the umbrella of a religion; but outside doctrine and practices.

The term "folk religion" is generally held to encompass two related but separate subjects. The first is the religious dimension of folk culture (folklore), or the folk-cultural dimensions of religion. The second refers to the study of religious syncretism between two cultures with different stages of formal expression, such as the melange of African folk beliefs and Roman Catholicism that led to the development of Vodun and Santería, and similar mixtures of formal religions with folk cultures. In China, folk Protestantism had its origins with the Taiping Rebellion.

Chinese folk religion, folk Hinduism, folk Christianity, and folk Islam are examples of folk religion associated with major religions. The term is also used, especially by the clergy of the faiths involved, to describe the desire of people who otherwise infrequently attend religious worship, do not belong to a church or similar religious society, and who have not made a formal profession of faith in a particular creed, to have religious weddings or funerals, or (among Christians) to have their children baptised.

In The Concise Oxford Dictionary of World Religions, John Bowker characterized "folk religion" as either "religion which occurs in small, local communities which does not adhere to the norms of large systems" or "the appropriation of religious beliefs and practices at a popular level."

Don Yoder argued that there were five separate ways of defining folk religion. The first was a perspective rooted in a cultural evolutionary framework which understood folk religion as representing the survivals of older forms of religion; in this, it would constitute "the survivals, in an official religious context, of beliefs and behaviour inherited from earlier stages of the culture's development". This definition would view folk religion in Catholic Europe as the survivals of pre-Christian religion and the folk religion in Protestant Europe as the survivals of Medieval Catholicism. The second definition identified by Yoder was the view that folk religion represented the mixture of an official religion with forms of ethnic religion; this was employed to explain the place of folk religion in the syncretic belief systems of the Americas, where Christianity had blended with the religions of indigenous American and African communities.

Yoder's third definition was that often employed within folkloristics, which held that folk religion was "the interaction of belief, ritual, custom, and mythology in traditional societies", representing that which was often pejoratively characterised as superstition. The fourth definition provided by Yoder stated that folk religion represented the "folk interpretation and expression of religion". Noting that this definition would not encompass beliefs that were largely unconnected from organised religion, such as in witchcraft, he therefore altered this definition by including the concept of "folk religiosity", thereby defining folk religion as "the deposit in culture of folk religiosity, the full range of folk attitudes to religion". His fifth and final definition represented a "practical working definition" that combined elements from these various other definitions. Thus, he summarized folk religion as "the totality of all those views and practices of religion that exist among the people apart from and alongside the strictly theological and liturgical forms of the official religion".

Yoder described "folk religion" as existing "in a complex society in relation to and in tension with the organized religion(s) of that society. Its relatively unorganized character differentiates it from organized religion".

Alternately, the sociologist of religion Matthias Zic Varul defined "folk religion" as "the relatively un-reflected aspect of ordinary practices and beliefs that are oriented towards, or productive of, something beyond the immediate here-and-now: everyday transcendence".