Phantasiasts (from Greek φαντασία, phantasia, appearance, phantasm) was a label applied to several distinct Christian heresies by their opponents in late antiquity. The term appears in Greek and Syriac writings mainly to refer to extreme forms of Miaphysitism. The term evokes the second-century heresy of Docetism. Both movements were accused of denying the full reality of Jesus's humanity.
The first targets of the label were the Eutychians, the followers of Eutyches. In a letter read before the Council of Chalcedon (451), Pope Leo the Great castigates the phantasmatici Christiani (Christian phantasmatics) in a clear reference to the Eutychians. Moderate Miaphysites like Timothy Aelurus, Philoxenus of Mabbug and Severus of Antioch also labelled the Eutychians phantasiasts. One Miaphysite oath administered to those returning to Miaphysitism from heresy called for the abjuration of the Phantasiasts. The use of the label Phantasiasts by both Dyophysites and moderate Miaphysites indicates the extreme nature of the position relative to orthodox theologies.