A protagonist (from Ancient Greek πρωταγωνιστής prōtagōnistḗs 'one who plays the first part, chief actor') is the main character of a story. The protagonist makes key decisions that affect the plot, primarily influencing the story and propelling it forward, and is often the character who faces the most significant obstacles and choices. If a story contains a subplot, or is a narrative made up of several stories, then each subplot may have its own protagonist.

The protagonist is the character whose fate is most closely followed by the reader or audience, and who is opposed by the antagonist. The antagonist provides obstacles and complications and creates conflicts that test the protagonist, revealing the strengths and weaknesses of the protagonist's character, and having the protagonist develop as a result. A particularly noble, virtuous, or accomplished protagonist is commonly called a hero, though the terms are not synonyms.

The term protagonist comes from Ancient Greek πρωταγωνιστής (prōtagōnistḗs) 'actor who plays the chief or first part', combined of πρῶτος (prôtos, 'first') and ἀγωνιστής (agōnistḗs, 'actor, competitor'), which stems from ἀγών (agṓn, 'contest') via ἀγωνίζομαι (agōnízomai, 'I contend for a prize').

The earliest known example of a protagonist can be found in the Epic of Gilgamesh, dated around the 3rd millennium BCE. However, the term originated only later with the dramatic performances of Ancient Greece. At first these involved merely dancing and recitation by the chorus, but then, according to Aristotle's Poetics, a poet named Thespis introduced the idea of one actor stepping out and engaging in a dialogue with the chorus. This was the invention of tragedy, and occurred about 536 B.C. Then the poet Aeschylus, in his plays, introduced a second actor, inventing the idea of dialogue between two characters. Sophocles then wrote plays that included a third actor.

A description of the protagonist's origin cited that during the early period of Greek drama, the protagonist served as the author, the director, and the actor and that these roles were only separated and allocated to different individuals later. There is also a claim that the poet did not assign or create the protagonist as well as other terms for actors such as deuteragonist and tritagonist primarily because he only gave actors their appropriate part. However, these actors were assigned their specific areas at the stage with the protagonist always entering from the middle door or that the dwelling of the deuteragonist (second most important character) should be on the right hand, and the tritagonist (third most important character), the left.

In Ancient Greece, the protagonist is distinguished from the term "hero", which was used to refer to a human who became a semi-divine being in the narrative.

In literary terms, a hero (masculine) or heroine (feminine) protagonist is typically admired for their achievements and noble qualities. Heroes are lauded for their strength, courage, virtuousness, and honor, and are considered to be the "good guys" of the narrative.

Examples include DC Comics' Superman (hero) and Katniss Everdeen from The Hunger Games (heroine).