Cleidemus of Athens was an Atthidographer, a historian of Athenian local history, active in the first half of the 4th century BCE. He is considered one of the earliest authors in this genre, a contemporary or near-contemporary of Hellanicus of Lesbos. No further biographical details survive.

His only known work is the Atthis, a chronicle of Athenian history, myth, and institutions. The work is lost, surviving only in fragments quoted by later authors.

Cleidemus holds significance as a foundational figure in Atthidography. His Atthis represents an early attempt to systematize Athenian tradition, covering events from the mythical past to at least the early 4th century BCE. He helped establish a model for local history later developed by Androtion and Philochorus, and his fragments provide valuable evidence for early Athenian historical writing and civic identity.