Pyrrho of Elis (/ˈpɪroʊ/; Ancient Greek: Πύρρων ὁ Ἠλεῖος; c. 360 – c. 270 BC) was a Greek philosopher of Classical antiquity, credited as being the first Greek skeptic philosopher and founder of Pyrrhonism.

Pyrrho of Elis is estimated to have lived from around 365/360 until 275/270 BCE. Pyrrho was from Elis, on the Ionian Sea. He was likely a member of the Klytidiai, a clan of seers in Elis who interpreted the oracles of the Temple of Zeus at Olympia where Pyrrho served as a high priest. The Klytidiai were descendants of Klytios, who was the son of Alcmaeon and the grandson of Amphiaraus. In the Python, Pyrrho's student Timon of Phlius describes first meeting Pyrrho on the grounds of an Amphiareion, i.e., a temple of Amphiaraus, while they were both on a pilgrimage to Delphi.

Most biographical information on Pyrrho, as well as some information concerning his demeanor and behavior, come from Diogenes Laertius; his work on Pyrrho's life drew primarily from the works of mid-third century BC biographer Antigonus of Carystus. Unlike the founders of other Hellenistic philosophies, Pyrrho was not substantively influenced by Socrates.

Pyrrho, along with Anaxarchus, travelled with Alexander the Great on his Indian campaign, "so that he even went as far as the Gymnosophists in India and the Magi" in Persia.

Pyrrho did not produce any written work. Most of the information on Pyrrho's philosophy comes from his student Timon. Only fragments of what Timon wrote have been preserved, mostly by Sextus Empiricus, Diogenes Laertius, and Eusebius. Little is known for certain about the details of Pyrrho's philosophy and how it may have differed from later Pyrrhonism. Most of what we know today as Pyrrhonism comes through the book Outlines of Pyrrhonism written by Sextus Empiricus over 400 years after Pyrrho's death.

Most sources agree that the primary goal of Pyrrho's philosophy was the achievement of a state of ataraxia, or freedom from mental perturbation, and that he observed that ataraxia could be brought about by eschewing beliefs (dogma) about thoughts and perceptions. However, Pyrrho's own philosophy may have differed significantly in details from later Pyrrhonism. Most interpretations of the information on Pyrrho's philosophy suggest that he claimed that reality is inherently indeterminate, which, in the view of Pyrrhonism described by Sextus Empiricus, would be considered a negative dogmatic belief.

A summary of Pyrrho's philosophy was preserved by Eusebius, quoting Aristocles, quoting Timon, in what is known as the "Aristocles passage." There are conflicting interpretations of the ideas presented in this passage, each of which leads to a different conclusion as to what Pyrrho meant:

'The things themselves are equally indifferent, and unstable, and indeterminate, and therefore neither our senses nor our opinions are either true or false. For this reason then we must not trust them, but be without opinions, and without bias, and without wavering, saying of every single thing that it no more is than is not, or both is and is not, or neither is nor is not.'