Marcion of Sinope (/ˈmɑːrkiən, -siən/; Ancient Greek: Μαρκίων Σινώπης; c. 85 – c. 160) was a theologian in early Christianity. Marcion preached that God had sent Jesus Christ, who was distinct from the "vengeful" God (Demiurge) who had created the world. He considered himself a follower of Paul the Apostle, whom he believed to have been the only true apostle of Jesus Christ; his doctrine is called Marcionism. Marcion published the earliest record of a canon of New Testament books.

Early Church writers such as Justin Martyr, Irenaeus, and Tertullian denounced Marcion as a heretic or antichrist, and he was excommunicated by the church of Rome around 144. He published his own canon of Christian sacred scriptures, which contained ten Pauline epistles (including the Epistle to the Laodiceans, while excluding the Pastoral epistles) and the Gospel of Marcion which historically is claimed to be an edited version of the Gospel of Luke. Some modern scholars, such as Matthias Klinghardt, have theorized that Marcion's Gospel was the oldest, although this has been contested.

This made Marcionism a catalyst in the process of the development of the New Testament canon by forcing the proto-orthodox Church to respond to his canon.

Epiphanius records in his Panarion (c. 375) that Marcion was born the son of a bishop in Pontus (modern-day Turkey), likely Philologus of Sinope. Rhodo and Tertullian, young men in Marcion's old age, described him as a "mariner" and a "ship-master" respectively. Some time in the late 130s, Marcion traveled to Rome, joined the Roman church, and made a large donation of 200,000 sesterces to the congregation there. Conflicts with the church of Rome arose and he was eventually excommunicated in 144, his donation being returned to him.

Irenaeus writes that "a certain Cerdo, originating from the Simonians, came to Rome under Hyginus [...] and taught that the one who was proclaimed as God by the Law and the Prophets is not the Father of our Lord Jesus Christ" (Against Heresies, 1, 27, 1). Also, according to them, Marcion and the Gnostic Valentinus were companions in Rome.

In 394, Epiphanius claimed that after beginnings as an ascetic, Marcion seduced a virgin and was accordingly excommunicated by his father, prompting him to leave his home town. Some scholars have taken this "seduction of a virgin" as a metaphor for Marcion's corruption of the Christian Church, with the Church portrayed as the undefiled virgin, and that Marcion apparently has become "the victim of the historicisation of such a metaphor, even though it contradicts the otherwise firm tradition of his strict sexual probity". Doubtful is Tertullian's claim in The Prescription Against Heretics (written c. 200) that Marcion professed repentance, and agreed to the conditions granted to him—that he should receive reconciliation if he restored to the Church those whom he had led astray—but that he was prevented from doing so by his death.

The Marcionite church expanded greatly within Marcion's lifetime, becoming a major rival to the other emerging church. After his death, it retained its following and survived Christian controversy and imperial disapproval for several centuries. Several theologians have viewed him as a proto-protestant.

Study of the Hebrew Bible, along with received writings circulating in the nascent Church, led Marcion to conclude that many of the teachings of Jesus were incompatible with the actions of Yahweh, characterized as the belligerent god of the Hebrew Bible. Marcion responded by developing a ditheistic system of belief around the year 144. This notion of two gods—a higher transcendent one and a lower world-creator and ruler—allowed Marcion to reconcile his perceived contradictions between Christian Covenant theology and the gospel proclaimed by the New Testament.