Ptolemy the Gnostic was a Christian theologian active in the Roman Empire during the late second century CE. He was a leading disciple of Valentinus and a major teacher within the Valentinian school, a significant branch of early Christian Gnosticism.

His role was to develop and systematize Valentinian teachings into a detailed cosmological framework. According to modern scholars, his system described a divine realm, called the Pleroma, populated by thirty emanations called Aeons, which explained the origin of the spiritual and material worlds. The primary source for his life and ideas is the anti-Gnostic work Against Heresies by Bishop Irenaeus of Lyon, written around 180 CE.

Only one of Ptolemy's own works survives: the Letter to Flora. This epistle, written to a Christian woman, outlines his method of interpreting the Bible. In it, he argues that the Law of Moses is not from one perfect God but is a composite from different sources, a view characteristic of Gnostic biblical analysis. This letter is exceptionally valuable as one of the few complete texts written by a Gnostic teacher from this period.

Ptolemy's historical importance lies in his role as a key systematizer of Valentinian thought, which was the most philosophically sophisticated form of Gnostic Christianity. His detailed teachings, preserved through his opponent Irenaeus, helped shape the early Christian church's doctrinal responses and debates about scripture, salvation, and the nature of God.