The Letters of Calanus are a collection of philosophical writings attributed to Calanus, an Indian sage from the 4th century BCE. The historical Calanus was a gymnosophist, or "naked philosopher," who traveled with Alexander the Great's army and famously died by self-immolation. He left no known writings, so the letters are a later pseudepigraphic work, meaning they were written by unknown authors in his name long after his death. Scholars date the composition of these letters to the Imperial Roman period, between the 1st and 6th centuries CE.

The letters belong to a popular genre of philosophical writing where teachings were attributed to famous wise figures to give them authority. In this case, Calanus was adopted as an ideal within the Cynic philosophical tradition. His legendary asceticism and fearless attitude toward death made him a useful model for Cynic ideas about simplicity, self-mastery, and rejecting luxury. The work survives only in fragments and mentions by later authors.

The significance of the Letters of Calanus lies in what they reveal about cultural exchange and literary practices in the ancient world. They show how Greco-Roman writers engaged with and reinterpreted the idea of "Indian wisdom." By creating these letters, later philosophers connected Cynic teachings to the prestige of an exotic sage, using his name to promote their own moral and ascetic views. The text is therefore a key example of how philosophical authority was constructed through pseudepigraphy in antiquity.