The Epistle to Diognetus is an early Christian text from the Roman era, written in Greek between the late second and early third century CE. Its author is unknown, and although it has been traditionally attributed to various well-known Christian figures, modern scholarship considers these attributions speculative. The author was a learned Christian apologist writing to a pagan named Diognetus, about whom nothing else is known.

The work is a single treatise in the form of a letter. It is a defense of Christian beliefs and practices, addressing misunderstandings from both pagan and Jewish perspectives. The text is celebrated for its eloquent description of the Christian community's role in the ancient world. According to modern scholars, a famous passage characterizes Christians as the "soul" of society—living within it as resident strangers, upholding their own spiritual laws while participating in civic life.

The letter's historical importance lies in its sophisticated insight into early Christian identity and its relationship with Greco-Roman culture. Its high literary quality and theological content have made it a valued document, though its anonymous and undated nature means its exact place in history is still discussed by academics. Two final chapters found in some manuscripts are now generally considered to be from a different, later work.