Theognetus was a Greek comic poet of the Hellenistic period in the 3rd century BCE. He wrote within the tradition of New Comedy, a style focused on everyday domestic and romantic narratives that flourished after the time of Aristophanes.

Virtually no details of his personal life are known. A later Byzantine encyclopedia notes he was the son of a man named Python, but no further biographical information survives. He is credited with two plays, both now lost: Philopatris (The Patriot) and Pythagorizousa (The Female Pythagorean). The latter title indicates his comedy engaged with contemporary intellectual life, likely featuring a woman who follows or pretends to follow the Pythagorean philosophical sect. Scholars see this as an example of New Comedy's practice of incorporating and gently satirizing current philosophical trends and social roles.

Theognetus's importance today rests on 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 exponents. His surviving fragments, though scant, provide a small glimpse into the language and themes of post-classical Greek theater.