Orphism is the name given to a set of religious beliefs and practices originating in the ancient Greek and Hellenistic world, associated with literature ascribed to the mythical poet Orpheus, who descended into the Greek underworld and returned. Orphism has been described as a reform of the earlier Dionysian religion, involving a re-interpretation or re-reading of the myth of Dionysus and a re-ordering of Hesiod's Theogony, based in part on pre-Socratic philosophy.

The suffering and death of the god Dionysus at the hands of the Titans has been considered the central myth of Orphism. According to this myth, the infant Dionysus is killed, torn apart, and consumed by the Titans. In retribution, Zeus strikes the Titans with a thunderbolt, turning them to ash. From these ashes, humanity is born. In Orphic belief, this myth describes humanity as having a dual nature: body (Ancient Greek: σῶμα, romanized: sôma), inherited from the Titans, and a divine spark or soul (Ancient Greek: ψυχή, romanized: psukhḗ), inherited from Dionysus. In order to achieve salvation from the Titanic, material existence, one had to be initiated into the Dionysian mysteries and undergo teletē, a ritual purification and reliving of the suffering and death of the god. The uninitiated (Ancient Greek: ἀμύητος, romanized: amúētos), they believed, would be reincarnated indefinitely.

Orphism is named after the legendary poet-hero Orpheus, who was said to have originated the Mysteries of Dionysus. However, Orpheus was more closely associated with Apollo than to Dionysus in the earliest sources and iconography. According to some versions of his mythos, he was the son of Apollo, and during his last days, he shunned the worship of other gods and devoted himself to Apollo alone.

Poetry containing distinctly Orphic beliefs has been traced back to the 6th century BC or at least 5th century BC, and graffiti of the 5th century BC apparently refers to "Orphics". The Derveni papyrus allows Orphic mythology to be dated to the end of the 5th century BC, and it is probably even older. Orphic views and practices are attested as by Herodotus, Euripides, and Plato. Plato refers to "Orpheus-initiators" (Ὀρφεοτελεσταί), and associated rites, although how far "Orphic" literature in general related to these rites is not certain.

Orphic views and practices have parallels to elements of Pythagoreanism, and various traditions hold that the Pythagoreans or Pythagoras himself authored early Orphic works; alternately, later philosophers believed that Pythagoras was an initiate of Orphism. The extent to which one movement may have influenced the other remains controversial. Some scholars maintain that Orphism and Pythagoreanism began as separate traditions which later became confused and conflated due to a few similarities. Others argue that the two traditions share a common origin and can even be considered a single entity, termed "Orphico-Pythagoreanism."

The belief that Pythagoreanism was a subset or direct descendant of Orphic religion existed by late antiquity, when Neoplatonist philosophers took the Orphic origin of Pythagorean teachings at face value. Proclus wrote:

In the fifteenth century, the Neoplatonic Greek scholar Constantine Lascaris (who found the poem Argonautica Orphica) considered a Pythagorean Orpheus. Bertrand Russell (1947) noted:

Study of early Orphic and Pythagorean sources, however, is more ambiguous concerning their relationship, and authors writing closer to Pythagoras' own lifetime never mentioned his supposed initiation into Orphism, and in general regarded Orpheus himself as a mythological figure. Despite this, even these authors of the 5th and 4th centuries BC noted a strong similarity between the two doctrines. In fact, some claimed that rather than being an initiate of Orphism, Pythagoras was actually the original author of the first Orphic texts. Specifically, Ion of Chios claimed that Pythagoras authored poetry which he attributed to the mythical Orpheus, and Epigenes, in his On Works Attributed to Orpheus, attributed the authorship of several influential Orphic poems to notable early Pythagoreans, including Cercops. According to Cicero, Aristotle also claimed that Orpheus never existed, and that the Pythagoreans ascribed some Orphic poems to Cercon (see Cercops).