Theognetus was a Greek comic poet who lived during the Hellenistic period in the 3rd century BCE. He wrote in the tradition of New Comedy, a style of theater that focused on everyday domestic and romantic stories, which flourished after the era of Aristophanes. Almost nothing is known about his personal life. A later Byzantine encyclopedia records that he was the son of a man named Python, but no other biographical details survive.

He is known to have written two plays, though both are now lost. Their titles are Philopatris ("The Patriot") and Pythagorizousa ("The Female Pythagorean"). The second title suggests his comedy engaged with contemporary intellectual life, as it likely featured a woman following or pretending to follow the Pythagorean philosophical sect. According to modern scholars, this points to New Comedy's practice of incorporating and gently satirizing current philosophical trends and social roles.

Theognetus's significance today lies in his status as one of many Hellenistic comic poets whose works are almost entirely lost. He represents the broad and active tradition of New Comedy that continued long after its most famous Athenian playwrights. His surviving fragments, though scant, offer a small, concrete glimpse into the language and themes of post-classical Greek theater.