Dionysius the Renegade was a Greek philosopher who lived in the late 4th and early 3rd centuries BCE. He was originally a member of the Cyrenaic school, which taught that pleasure is the highest good in life. He studied under Aristippus the Younger, a grandson of the school's founder.

His famous nickname, "the Renegade," comes from his dramatic change of philosophical allegiance. According to ancient accounts, Dionysius suffered from a severe eye disease. Finding the Cyrenaic focus on pleasure inadequate to cope with his chronic pain, he abandoned the school and joined the rival Megarian school, which emphasized logical argument and a more austere way of life.

He is known to have written a work titled On Apathy, which is now lost. He composed this text to argue for the possibility of achieving a state free from passion and pain, directly opposing his former Cyrenaic teachings.

Modern scholars see Dionysius as a minor but illustrative figure in Hellenistic philosophy. His personal story highlights how practical life experiences could drive philosophical change in antiquity. His defection and his work on apathy demonstrate the active debates and exchange of ideas between competing schools of thought during this period.