Valentinus (also spelled Valentinius) (c.100 - c.160) is the best known and for a time most successful early Christian gnostic theologian.

He founds his school in Rome.

According to Tertullian, Valentinus is a candidate for bishop but started his own group when another was chosen.

Valentinus produces a variety of writings, but only fragments survive, largely those embedded in refuted quotations in the works of his opponents, not enough to reconstruct his system except in broad outline.

His doctrine is known to us only in the developed and modified form given to it by his disciples.

He teaches that there are three kinds of people, the spiritual, psychical, and material; and that only those of a spiritual nature (his own followers) receive the gnosis (knowledge) that allows them to return to the divine Pleroma, while those of a psychic nature (ordinary Christians) will attain a lesser form of salvation, and that those of a material nature (pagans and Jews) are doomed to perish.

Valentinus has a large following, the Valentinians.

It later divides into an Eastern and a Western or Italian branch.

The Marcosians belong to the Western branch.