Diogenes of Sinope was a Greek philosopher of the 4th century BCE and the most famous practitioner of Cynicism. He was born in the city of Sinope on the Black Sea and died in Corinth in 323 BCE. According to ancient accounts, he was exiled from his hometown, possibly for financial misconduct involving currency, an act he later reinterpreted as a metaphor for his life's work: to "deface the coinage" of societal conventions.

He moved to Athens, where he embraced an extreme life of poverty and self-sufficiency. He famously lived in a large storage jar, owned only a cloak, staff, and bag, and performed public acts intended to shame societal norms and provoke philosophical reflection. His defiant, "shameless" behavior earned him the nickname "the Dog" (from which the term "Cynic" is derived). Later in life, after a period of enslavement, he lived in Corinth, where he served as a tutor.

No writings confirmed to be by Diogenes survive. Ancient sources list titles of works attributed to him, like a Republic, but scholars widely consider these to be spurious or lost. His philosophy and legendary status are known entirely from anecdotes and sayings preserved by later authors.

Diogenes is the archetypal Cynic philosopher. His life modeled the core Cynic ideals of living in accordance with nature, radical self-sufficiency, and using frank criticism to challenge wealth, power, and social pretense. His influence was significant, particularly on the later Stoic school, which adopted but moderated his ascetic principles. He remains a powerful symbol of philosophical critique and anti-conventionalism.