Marcus was the founder of the Marcosian Gnostic sect in the 2nd century AD. He was a disciple of Valentinus, with whom his system mainly agrees. His doctrines are almost exclusively known to us through a long polemic (i. 13–21) in Adversus Haereses, in which Irenaeus gives an account of his teaching and his school. Clement of Alexandria clearly knew of Marcus and actually used his number system (Stromata, VI, xvi), though without acknowledgement.