Axiothea of Phlius (Greek: Ἀξιοθέα Φλειασία fl. c. 350 BCE) was a female student of Plato and Speusippus.
She was born in Phlius, an ancient city in the Peloponnese which was under Spartan rule when Plato founded his Academy. Axiothea is said by Themistius to have read Plato's Republic and then travelled to Athens to be his student. In order to avoid becoming a hetaera, Axiothea dressed as a man during her time at Plato's Academy. After the death of Plato she continued her studies with Speusippus, Plato's nephew.
A papyrus fragment from Oxyrhynchus mentions an unidentified woman who studied under Plato, Speusippus, and then Menedemus of Eretria. The fragment goes on to explain that "in her teens she was lovely and full of unstudied grace." This woman is probably Axiothea or Lastheneia of Mantinea.
Axiothea appears as a character in Mary Renault's novel The Mask of Apollo.