In linguistics, grammatical relations (also called grammatical functions, grammatical roles, or syntactic functions) are functional relationships between constituents in a clause. The standard examples of grammatical functions from traditional grammar are subject, direct object, and indirect object. In recent times, the syntactic functions (more generally referred to as grammatical relations), typified by the traditional categories of subject and object, have assumed an important role in linguistic theorizing, within a variety of approaches ranging from generative grammar to functional and cognitive theories. Many modern theories of grammar are likely to acknowledge numerous further types of grammatical relations (e.g. complement, specifier, predicative, etc.).

The role of grammatical relations in theories of grammar is greatest in dependency grammars, which tend to posit dozens of distinct grammatical relations. Every head-dependent dependency bears a grammatical function.

Grammatical categories are assigned to the words and phrases that have the relations. This includes traditional parts of speech like nouns, verbs, adjectives, etc., and features like number and tense.

The grammatical relations are exemplified in traditional grammar by the notions of subject, direct object, and indirect object:

The subject Fred performs or is the source of the action. The direct object the book is acted upon by the subject, and the indirect object Susan receives the direct object or otherwise benefits from the action. Traditional grammars often begin with these rather vague notions of the grammatical functions. When one begins to examine the distinctions more closely, it quickly becomes clear that these basic definitions do not provide much more than a loose orientation point.

What is indisputable about the grammatical relations is that they are relational. That is, subject and object can exist as such only by virtue of the context in which they appear. A noun such as Fred or a noun phrase such as the book cannot qualify as subject and direct object, respectively, unless they appear in an environment, e.g. a clause, where they are related to each other and/or to an action or state. In this regard, the main verb in a clause is responsible for assigning grammatical relations to the clause "participants".

Most grammarians and students of language intuitively know in most cases what the subject and object in a given clause are. But when one attempts to produce theoretically satisfying definitions of these notions, the results are usually less clear and therefore controversial. The contradictory impulses have resulted in a situation where most theories of grammar acknowledge the grammatical relations and rely on them heavily for describing phenomena of grammar but at the same time, avoid providing concrete definitions of them. Nevertheless, various principles can be acknowledged that attempts to define the grammatical relations are based on.

The thematic relations (also known as thematic roles, and semantic roles, e.g. agent, patient, theme, goal) can provide semantic orientation for defining the grammatical relations. There is a tendency for subjects to be agents and objects to be patients or themes. However, the thematic relations cannot be substituted for the grammatical relations, nor vice versa. This point is evident with the active-passive diathesis and ergative verbs: