Theagenes (Greek: Θεαγένης; fl. c. 160 AD) of Patras, was a Cynic philosopher and close friend of Peregrinus Proteus.

He is known principally as a character who appears in Lucian's The Death of Peregrinus (Latin: De Morte Peregrini), where he is introduced as praising Peregrinus' desire to kill himself by self-immolation:

Proteus," he cried, "Proteus vain-glorious? Who dares name the word? Earth! Sun! Seas! Rivers! God of our fathers, Heracles! Was it for this that he suffered bondage in Syria? that he forgave his country a debt of a million odd? that he was cast out of Rome, — he whose brilliance exceeds the Sun, fit rival of the Lord of Olympus? 'Tis his good will to depart from life by fire, and they call it vain-glory! What other end had eracles?^[1]

Theagenes aided Peregrinus by setting fire to the funeral pyre, which Peregrinus threw himself upon.^[2] Theagenes compared Peregrinus' suicide with the similar practices of the Gymnosophists in India.^[3] Lucian, who was hostile to Theagenes and his brand of Cynicism, states that Theagenes had a fortune of fifteen talents, which he obtained by money-lending.^[4]

Theagenes is also mentioned by Galen, who recalls how he lectured daily to large crowds in Trajan's Forum in Rome.^[5] Galen describes how Theagenes died when he was given an inappropriate treatment by Statilius Attalus (physician to Marcus Aurelius) for inflammation of the liver.^[5]