Antisthenes was an Athenian philosopher who lived from approximately 445 to 365 BCE, during the Classical period. He is considered a foundational figure for the Cynic school of philosophy. His father was Athenian, but his mother was a Thracian slave, a status that made him a nothos (a person of mixed birth) and likely restricted his full civic rights. He first studied under the sophist Gorgias and taught rhetoric before becoming a devoted follower of Socrates, even being present at Socrates's death. After 399 BCE, he founded his own school at the Cynosarges gymnasium, a site associated with nothoi, where he taught, most notably, Diogenes of Sinope.

Ancient sources report he was a prolific writer. According to later biographers like Diogenes Laertius, he authored over sixty works on ethics, logic, theology, and literature. However, none of his complete writings survive today; his ideas are known only through fragments and reports by later authors.

According to modern scholars, his primary significance lies in forming a direct link between Socrates and the Cynic tradition. His philosophy emphasized virtue, defined as practical wisdom and strength of character, over theoretical knowledge. He advocated for an ascetic life, holding that virtue alone was sufficient for happiness and that hardship was a path to moral strength. While later Cynics developed his ideas into a more radical lifestyle, Antisthenes is credited with establishing the core Socratic-inspired ethical foundation for the movement.