Diogenes ( dy-OJ-in-eez; Ancient Greek: Διογένης, romanized: Diogénēs [di.oɡénɛːs]), also known as Diogenes the Cynic (Διογένης ὁ Κυνικός, Diogénēs ho Kynikós) or Diogenes of Sinope, was a Greek philosopher and one of the founders of Cynicism. He was born in Sinope, an Ionian colony on the Black Sea coast of Anatolia in 412 or 404 BC and died at Corinth in 323 BC.

Diogenes was a controversial figure. He was allegedly banished, or fled from, Sinope for debasement of currency. He was the son of the mintmaster of Sinope, and there is some debate as to whether or not he alone had debased the Sinopian currency, whether his father had done this, or whether they had both done it. After his hasty departure from Sinope he moved to Athens where he proceeded to criticize many conventions of Athens of that day. There are many tales about his dogging Antisthenes' footsteps and becoming his "faithful hound". Diogenes was captured by pirates and sold into slavery, eventually settling in Corinth. There he passed his philosophy of Cynicism to Crates, who taught it to Zeno of Citium, who fashioned it into the school of Stoicism, one of the most enduring schools of Greek philosophy.

No writings of Diogenes survive, but there are some details of his life from anecdotes (chreia), especially from Diogenes Laërtius' book Lives and Opinions of Eminent Philosophers and some other sources. Diogenes made a virtue of poverty. He begged for a living and often slept in a large ceramic jar, or pithos, in the marketplace. He used his simple lifestyle and behavior to criticize the social values and institutions of what he saw as a corrupt, confused society. He had a reputation for sleeping and eating wherever he chose in a highly non-traditional fashion and took to toughening himself against nature. He declared himself a cosmopolitan and a citizen of the world rather than claiming allegiance to just one place.

He modeled himself on the example of Heracles, believing that virtue was better revealed in action than in theory. He became notorious for his philosophical stunts, such as carrying a lamp during the day, claiming to be looking for a "man" (often rendered in English as "looking for an honest man"). He criticized Plato, disputed his interpretation of Socrates, and sabotaged his lectures, sometimes distracting listeners by bringing food and eating during the discussions. Diogenes was also noted for having mocked Alexander the Great, both in public and to his face when he visited Corinth in 336 BC.