Semi-Pelagianism (or Semipelagianism) is a historical Christian theological and soteriological school of thought about the role of free will in salvation. In semi-Pelagian thought, a distinction is made between the beginning of faith and the increase of faith. Semi-Pelagian thought teaches that the latter half – growing in faith – is the work of God, while the beginning of faith is an act of free will, with grace supervening only later.

The term "semi-Pelagianism", a 16th-century coinage, is considered a misnomer by many modern scholars. "Semi-Pelagianism" has frequently been used in a pejorative sense.

Semi-Pelagianism was, in the theory, originally developed as a compromise between Pelagianism and the teaching of Church Fathers such as Saint Augustine. Adherents to Pelagianism hold that people are born untainted by sin and do not need salvation unless they choose to sin, a belief which had been dismissed as heresy. In contrast, Augustine taught that people cannot come to God without the grace of God. Like Pelagianism, what is now called semi-Pelagianism was labeled heresy by the Western Church at the Second Council of Orange in 529.

In contrast, most Christian communions teach that the initiative for faith comes from God. Some, notably Catholics and Orthodox, teach that it then requires free collaboration on the part of man (synergism): "The fatherly action of God is first on his own initiative, and then follows man's free acting through his collaboration". "Since the initiative belongs to God in the order of grace, no one can merit the initial grace of forgiveness and justification at the beginning of conversion. Moved by the Holy Spirit and by charity, we can then merit for ourselves and for others the graces needed for our sanctification, for the increase of grace and charity, and for the attainment of eternal life."

Pelagianism is the teaching that people have the capacity to seek God in and of themselves apart from any movement of God or the Holy Spirit, and therefore that salvation is effected by their own efforts. The doctrine takes its name from Pelagius, a British monk who was accused of developing the doctrine (he himself appears to have claimed in his letters that man does not do good apart from grace, claiming only that all men have free will by God's gift); it was opposed especially by Augustine of Hippo and was declared a heresy by Pope Zosimus in 418. Rejecting the existence of original sin, it teaches that man is in himself and by nature capable of choosing good.

In so-called semi-Pelagian thought, both God and the human person always participate in the salvation process. Humans make free will choices, which are aided by God through creation, natural grace, "supernatural" grace, God's restrictions on demonic invasion; God continually brings the human person to real choices, which God also aids, in the process of spiritual growth to be saved. The entire process is grace; snapshot focus on the specific moments of decision are always in the context of the overarching grace of God. Semi-Pelagianism is similar to synergism, which is the traditional patristic doctrine.

The term "semi-Pelagianism" was unknown in antiquity, appearing for the first time only in the last quarter of the 16th century in connection with Luis de Molina's doctrine of grace: opponents of this theologian believed they saw a close resemblance to the views advocated by monks of Southern Gaul at and around Marseille after 428.

Even after this confusion between the ideas of Molina and those of the monks of Marseille had been exposed as an error, the newly coined term "semi-Pelagianism" was retained in learned circles as an apt designation for the views of those monks, most notably John Cassian, which were said to have aimed at a compromise between Pelagianism and Augustinism. It was condemned as heresy at the local Council of Orange (529) after disputes extending over more than a hundred years. The monks, however, consistent with the Desert Fathers, considered their teaching to be the ancient teaching of the Church.