Theodectes of Phaselis was a 4th-century BCE Greek rhetorician and tragic poet from Phaselis in Lycia. He studied under Isocrates, Plato, and Aristotle in Athens and was a contemporary of Aristotle and Alexander the Great. Ancient sources record his success, noting seven or eight victories in dramatic competitions, and his skill as an orator who wrote a rhetorical handbook. An anecdote in the Suda claims he died at 41 after choking on a grape stone, though this is dubious.

Though surviving only in fragments, Theodectes was a prolific author credited with approximately 50 tragedies. Known titles include Alcmaeon, Helen, Lynceus, Oedipus, and Philoctetes. His tragedy Mausolus was likely composed for the Carian dynast Mausolus, builder of the Mausoleum at Halicarnassus, suggesting a connection to his court. He also wrote an encomium to Heracles.

Theodectes is significant for bridging rhetoric and tragedy in the 4th century BCE. His training under both Isocrates and Aristotle placed him at a key intellectual intersection. Aristotle frequently cites his plays in the Poetics and Rhetoric as examples of tragic technique and rhetorical argument, cementing his importance as a case study in applying rhetorical theory to tragic practice.