Diagoras "the Atheist" of Melos (Greek: Διαγόρας ὁ Μήλιος) was a Greek poet and sophist of the 5th century BC. Throughout Antiquity he was regarded as an atheist. With the exception of this one point, there is little information concerning his life and beliefs. He spoke out against the Greek religion, and criticized the Eleusinian Mysteries. The Athenians accused him of impiety, and he was forced to flee the city. He died in Corinth.
Diagoras was the son of Telecleides or Teleclytus, and was born in the island of Melos, one of the Cyclades. According to the Suda, he was a disciple of Democritus after Democritus had paid 10,000 drachmas to free Diagoras from captivity following the cruel subjugation of Melos (416 BC); however no early sources mention an association with Democritus. The Suda also states that in his youth Diagoras had acquired some reputation as a lyric poet, and this is probably the cause of his being mentioned with the lyric poets Simonides, Pindar, and Bacchylides. Among his encomia is mentioned in particular a eulogy on Arianthes of Argos, who is otherwise unknown, another on Nicodorus, a statesman of Mantineia, and a third upon the Mantineians. Nicodorus was celebrated as a statesman and lawgiver in his native place; Aelian informs us that Diagoras was the lover of Nicodorus, and assisted Nicodorus in his legislation. The constitution of Mantineia was later praised by Aristotle and Polybius as a rare example of democractic moderation and equilibrium.