In linguistics, a grammatical agent is an entity that carries out an action. For example, in the sentence "Jack kicked the ball", Jack is the agent. In certain languages, the agent is declined or otherwise marked to indicate its grammatical role. In Japanese, for instance, the agent is typically affixed with the hiragana が (pronounced "ga"). Although Modern English does not mark grammatical role, agency is informally represented using certain conventions; for instance, with the morphemes "-ing", "-er", or "-or", as in "eating", "user", or "prosecutor".

The grammatical agent is often confused with the subject, but there is a slight difference: the former is based explicitly on its relationship to the verb, whereas the latter is based on its prominence in the sentence. To illustrate: in the passive sentence "The cake is eaten by Maria", the cake is the subject, and Maria is the agent. However, in most sentences, the agent and the subject tend to match.