Eutychianism refers to a set of Christian theological doctrines derived from the ideas of Eutyches of Constantinople (c. 380—456). Eutychianism is a specific understanding of how the human and divine relate within the person of Jesus Christ (that is, Eutychianism is a Christology).

At various times, Eutyches taught that the human nature of Christ was overcome by the divine, or that Christ had a human nature but it was unlike the rest of humanity. One formulation is that Eutychianism stressed the unity of Christ's nature to such an extent that Christ's divinity consumed his humanity as the ocean consumes a drop of vinegar. Eutyches maintained that Christ was of two natures but not in two natures: separate divine and human natures had united and blended in such a manner that although Jesus was homoousian with the Father, he was not homoousian with man.

The interpretation has been regarded as monophysite or miaphysite.

The response to Eutychianism resulted in the Fourth Ecumenical Council in Chalcedon in 451, and the statement of faith known as the Chalcedonian Creed. The reaction against Eutychianism also led to the schism with Oriental Orthodoxy.