Cantharus was an Athenian comic poet active in the 5th century BCE during the period known as Old Comedy. He is a relatively obscure figure today, but ancient records confirm he was a recognized competitor in Athens's major dramatic festivals. According to the Parian Marble, an ancient inscription, he won his first victory at the City Dionysia in 422 BCE. A later Byzantine source provides a different date for the peak of his career, but modern scholars generally consider the earlier record more reliable. This places him as a contemporary of more famous playwrights like Aristophanes.

His works have not survived intact. He is credited with two known play titles: Tereus and Alcmaeon. Both were comedies based on mythological figures, a common practice in Old Comedy known as mythological burlesque. A later source attributes six plays to him, but only these two titles are specifically attested elsewhere. What remains of his work consists only of fragments and references quoted by later authors.

Cantharus represents the many successful comic poets of his era whose plays are now lost. His recorded victory demonstrates the competitive and vibrant nature of Athenian theatre, which featured many playwrights beyond the few whose complete works survive. According to modern scholars, the fragments and records related to Cantharus help historians reconstruct the broader history and thematic range of Old Comedy.