in ancient Greek drama, the first or leading actor. The poet Thespis is credited with having invented tragedy when he introduced this first actor into Greek drama, which formerly consisted only of choric dancing and recitation. The protagonist stood opposite the chorus and engaged in an interchange of questions and answers. According to Aristotle in his Poetics, Aeschylus brought in a second actor, or deuteragonist, and presented the first dialogue between two characters. Aeschylus' younger rival, Sophocles, then added a third actor, the tritagonist, and was able to write more complex, more natural dialogue. That there were only three actors did not limit the number of characters to three because one actor would play more than one character.

In the early days of Greek drama, the dramatists chose and often trained their own actors. By 449 BC, however, the leading actors were chosen by the chief magistrates of Athens, the archons. These leading actors, the protagonists, were responsible for selecting the supporting actors, the deuteragonists and tritagonists. The protagonists also competed for acting prizes that were independent of the contests for the best tragedies. The term protagonist has come to be used for the principal character in a novel, story, or drama.