Phrynichus was an Athenian comic playwright active in the late 5th century BCE, during the Classical period. He is a distinct figure from an earlier tragic poet who shared the same name. Phrynichus wrote in the genre of Old Comedy, a style known for its bold satire and fantastical stories, and was a contemporary of the more famous Aristophanes. According to ancient records, he produced his first play around 429 BCE and won prizes at major Athenian festivals, including a first prize at the City Dionysia.

His exact birth and death dates are unknown, and few personal details about his life survive. Scholars reconstruct his career from surviving play titles, ancient commentaries, and later historical mentions. He was a recognized competitor in Athens' vibrant theatrical culture, which flourished even during the Peloponnesian War.

Only fragments and titles of his comedies survive. Ancient sources disagree on the total number of plays he wrote. His known works include Monotropos (performed in 414 BCE), Muses (performed in 405 BCE), and others such as Revelers and Tragedians. The attribution of some titles is uncertain, and they may represent alternate names for the same play.

Phrynichus is significant as one of the attested but lesser-known poets of Old Comedy, demonstrating the genre's competitive breadth beyond its most famous authors. A notable historical fact is that his play Muses defeated Aristophanes' famous comedy Frogs for first prize in 405 BCE, showing that contemporary judgments sometimes differed from later literary fame. The fragments of his work help modern scholars understand the common themes, language, and stylistic range of Athenian Old Comedy.