Alcidamas was a Greek teacher of rhetoric and a sophist from the late 5th to early 4th century BCE. He was a student of the famous sophist Gorgias and taught in Athens, where he was a contemporary and rival of the orator Isocrates. His career represents the traveling professional sophists of the Classical period who offered instruction in persuasive speaking.

His surviving work is limited. The most complete text is a speech titled "On the Sophists," which argues for the superiority of skilled, improvisational speaking over delivering pre-written speeches. A second work, a fragmentary declamation called "Odysseus" (or "Against the Treason of Palamedes"), also survives. Other works attributed to him are lost.

Alcidamas is historically important for his role in a major debate about rhetorical practice. According to modern scholars, his treatise "On the Sophists" is a key document defending spontaneity and flexibility in oratory as more authentic for real political and legal situations. His fragmentary "Odysseus" speech is seen as an early example of a mythological exercise that became standard in later rhetorical training. His arguments about the limits of written texts also provide an interesting contrast to philosophical critiques of writing from the same era.