The Brethren of Purity (Arabic: إخوان‌ الصفا, romanized: ʾIkhwān aṣ-Ṣafā; also The Brethren of Sincerity) were a secret society of Muslim philosophers in Basra, Iraq, in the 9th or 10th century CE.

The structure of the organization and the identities of its members have never been clear. Their esoteric teachings and philosophy are expounded in an epistolary style in the Encyclopedia of the Brethren of Purity (Rasā'il Ikhwān al-Ṣafā'), a giant compendium of 52 epistles that would greatly influence later encyclopedias. A good deal of Muslim and Western scholarship has been spent on just pinning down the identities of the Brethren and the century in which they were active.

The Arabic phrase Ikhwān aṣ-Ṣafāʾ (short for, among many possible transcriptions, Ikhwān aṣ-Ṣafāʾ wa Khullān al-Wafā wa Ahl al-Ḥamd wa abnāʾ al-Majd, meaning "Brethren of Purity, Loyal Friends, People worthy of praise and Sons of Glory") can be translated as either the "Brethren of Purity" or the "Brethren of Sincerity"; various scholars such as Ian Netton prefer "of Purity" because of the group's ascetic impulses towards purity and salvation.

A suggestion made by Ignác Goldziher, and later written about by Philip Khuri Hitti in his History of the Arabs, is that the name is taken from a story in Kalilah waDimnah, in which a group of animals, by acting as faithful friends (ikhwān aṣ-ṣafāʾ), escape the snares of the hunter. The story concerns a Barbary dove and its companions who become entangled in the net of a hunter trapping birds. Together, they betake themselves, still entangled in the net, to a nearby rat, who is gracious enough to gnaw the birds free of the cords in which they have become enmeshed. Impressed by the rat's altruistic deed, a crow strikes up a friendship with him. Soon afterward a tortoise and gazelle also join the company of animals. Some time later, the gazelle becomes trapped in another net; but with the aid of the kind rat and the other animals, it is soon freed. The tortoise, however, is too slow in making his escape and finds himself captured by the hunter. In the final turn of events, the gazelle repays the tortoise by serving as a decoy and distracting the hunter while the rat and the others free the tortoise. After this, the animals are designated as the "Ikhwān aṣ-Ṣafāʾ".

This story is mentioned as an exemplum when the Brethren speak of mutual aid in one risāla (epistle), a crucial part of their system of ethics that has been summarized thus:

In this Brotherhood, self is forgotten; all act by the help of each, all rely upon each for succour and advice, and if a Brother sees it will be good for another that he should sacrifice his life for him, he willingly gives it.

The Brethren regularly met on a fixed schedule. The meetings apparently took place on three evenings of each month: once near the beginning, in which speeches were given, another towards the middle, apparently concerning astronomy and astrology, and the third between the end of the month and the 25th of that month; during the third one, they recited hymns with philosophical content. During their meetings and possibly also during the three feasts they held, on the dates of the sun's entry into the Zodiac signs "Ram, Cancer, and Balance" (which doubled as the March equinox, summer solstice, and September equinox), beyond the usual lectures and discussions, they would engage in some manner of liturgy reminiscent of the Sabians of Harran.

Hierarchy was a major theme in the Encyclopedia, and unsurprisingly, the Brethren loosely divided themselves up into four ranks by age; the age guidelines would not have been firm: for example, such an exemplar of the fourth rank as Jesus would have been too young if the age guidelines were absolute and fixed. Compare the similar division of the Encyclopedia into four sections and the Jabirite symbolism of 4. The ranks were: