The Basilidians or Basilideans /ˌbæsɪˈlɪdiənz, ˌbæz-/ were a Gnostic sect founded by Basilides of Alexandria in the 2nd century. Basilides claimed to have been taught his doctrines by Glaucus, a disciple of St. Peter, though others stated he was a disciple of the Simonian Menander.

Basilides enjoined on his followers, like Pythagoras, a silence of five years. They kept the anniversary of the day of the baptism of Jesus as a feast day and spent the eve of it in reading. Basilides also instructed his followers not to scruple eating things offered to idols. The sect had three grades – material, intellectual and spiritual – and possessed two allegorical statues, male and female. The sect's doctrines were often similar to those of the Ophites and later Jewish Kabbalah.

Basilidianism survived until the end of the 4th century as Epiphanius knew of Basilidians living in the Nile Delta. It was however almost exclusively limited to Egypt, though according to Sulpicius Severus it seems to have found an entrance into Spain through a certain Mark from Memphis. Jerome was of the opinion that Priscillian, the founder of Priscillianism, was influenced by "the heresy of Basilides".

The descriptions of the Basilidian system given by our chief informants, Irenaeus (Adversus Haereses) and Hippolytus (Philosophumena), are so strongly divergent that they seem to many quite irreconcilable. According to Hippolytus, Basilides was apparently a pantheistic evolutionist; and according to Irenaeus, a dualist and an emanationist. Historians such as Philip Schaff have the opinion that "Irenaeus described a form of Basilidianism which was not the original, but a later corruption of the system. On the other hand, Clement of Alexandria surely, and Hippolytus, in the fuller account of his Philosophumena, probably drew their knowledge of the system directly from Basilides' own work, the Exegetica, and hence represent the form of doctrine taught by Basilides himself".

The fundamental theme of the Basilidian system is the question concerning the origin of evil and how to overcome it. A cosmographical feature common to many forms of Gnosticism is the idea that the Logos Spermatikos is scattered into the sensible cosmos, where it is the duty of the Gnostics, by whatever means, to recollect these scattered seed-members of the Logos and return them to their proper places (cf. the Gospel of Eve). "Their whole system," says Clement, "is a confusion of the Panspermia (All-seed) with the Phylokrinesis (Difference-in-kind) and the return of things thus confused to their own places."

According to Hippolytus, Basilides asserted the beginning of all things to have been pure nothing. He uses every device of language to express absolute nonentity. Nothing then being in existence, "not-being God" willed to make a not-being world out of not-being things. This not-being world was only "a single seed containing within itself all the seed-mass of the world," as the mustard seed contains the branches and leaves of the tree. Within this seed-mass were three parts, or sonships, and were consubstantial with the not-being God. This was the one origin of all future growths; these future growths did not use pre-existing matter, but rather these future growths came into being out of nothing by the voice of the not-being God.

Part subtle of substance. The first part of the seed-mass burst through and ascended to the not-being God.

Part coarse of substance. The second part of the seed-mass to burst forth could not mount up of itself, but it took to itself as a wing of the Holy Spirit, each bearing up the other with mutual benefit. But when it came near the place of the first part of the seed-mass and the not-being God, it could take the Holy Spirit no further, it not being consubstantial with the Holy Spirit. There the Holy Spirit remained, as a firmament dividing things above the world from the world itself below.