Agonist (from Ancient Greek ἀγωνιστής: agōnistēs, “combatant, champion”) may refer to:
- A person engaged in a contest or struggle (see agon)
- An advocate of the political theory of agonism (or "agonistic pluralism")
- Agonist, a substance that binds to a receptor to induce a biochemical response
- Inverse agonist, a substance that induces the opposite effect of an agonist
- Agonist (muscle), a muscle type
- The Agonist, a Canadian metal band
- Agonistic behaviour, animal behaviour relating to conflict
