Cleidemus of Athens was a 5th-century BCE Athenian writer, recognized as the earliest known Atthidographer—a composer of local histories of Attica. His work initiated the tradition of Athenian local historiography. Beyond this authorship, no secure biographical details survive; his dating is inferred from his foundational position in the genre preceding figures like Hellanicus.

His sole known work is the Atthis, a history of Attica from mythical times to at least the late 5th century BCE. The text is lost, preserved only in fragments cited by later authors. Cleidemus holds significance as the founder of Atthidography. His Atthis established a model for subsequent historians, creating a crucial genre focused on Athenian local traditions, cults, and institutions, which served as a counterpoint to pan-Hellenic history. The fragments cited in later sources provide valuable evidence for early Athenian history and mythology.