Castalia /kəˈsteɪliə/ (Ancient Greek: Κασταλία), in Greek mythology, was a naiad-nymph, a daughter of Achelous who inhabited the Castalian spring in Delphi. In older traditions, Castalian spring already existed by the time Apollo came to Delphi searching for Python.[1] According to some, the water was a gift to Castalia from the river Cephisus.

Latin poet Lactantius Placidus in his commentary on Statius’ Thebaid tells that to escape Apollo’s amorous advances, Castalia transformed herself into a fountain at Delphi, at the base of Mount Parnassus, or at Mount Helicon.[2] She inspired the genius of poetry to those who drank her waters or listened to their quiet sound; the sacred water was also used to clean the Delphian temples. Apollo consecrated Castalia to the Muses (Castaliae Musae).