Speusippus was an Athenian philosopher of the 4th century BCE. He was the nephew of Plato and a lifelong member of Plato's Academy. Upon Plato's death in 347 BCE, Speusippus succeeded his uncle as the second head of the school, a position he held for eight years. His tenure involved philosophical disputes, notably with Aristotle, who left the Academy around this time. According to ancient accounts, Speusippus suffered from a severe illness and died by suicide.

He was a prolific writer, but none of his works survive complete. Modern knowledge of his ideas comes from fragments and reports in later philosophers like Aristotle and Diogenes Laertius. His writings covered many subjects, including metaphysics, mathematics, biology, and ethics. Known titles include On Pythagorean Numbers, On Pleasure, and Similar Things, a work on biological classification.

Speusippus is historically significant as the immediate successor who ensured the Academy's continuity after its founder. His philosophical importance lies in his systematic development of Platonic doctrine, though he also introduced his own theories. According to modern scholars, he developed a metaphysical system where a supreme principle called "the One" was the source of all reality, but he separated this principle from the Platonic Idea of the Good—a departure criticized by Aristotle. His work represents a crucial, though fragmentary, link between Plato and the later developments of Hellenistic philosophy.