Theodorus of Gadara (Greek: Θεόδωρος ὁ Γαδαρεύς) was a Greek rhetorician of the 1st century BC who founded a rhetorical school in Gadara (present-day Um Qais, Jordan), where he taught future Roman emperor Tiberius the art of rhetoric. It was written of Tiberius that:
...even in his boyhood, his cruel and cold nature did not lie hidden. Theodorus of Gadara was his teacher of rhetoric and, in all his wisdom, seems to have been the first to have understood Tiberius and to have capped him with a very pithy saying when he taunted Tiberius, calling him 'Mud kneaded with blood'... .His other well-known pupil was Greek rhetorician Hermagoras of Temnos, who later taught oratory in Rome.
Theodorus wrote the following books, among others: On Questions in Pronunciation (3 books), On History (1 book), On Thesis (1 book), On the Similarity of Dialects and its Demonstration (2 books), On the Constitution (2 books), On Coele Syria (1 book), and On the Capacity of the Orator (1 book).
Theodorus was one of the two most famous rhetoric teachers of the time, the other being Apollodorus of Pergamon. Students of Apollodorus were commonly referred to as Apollodoreans, while students of Theodorus were known as Theodoreans.