A complementizer, as used in linguistics (especially generative grammar), is a syntactic category (part of speech) roughly equivalent to the term subordinating conjunction in traditional linguistics. For example, the word that is generally called a complementizer in English sentences like Mary believes that it is raining.

The standard abbreviation for complementizer is C. The complementizer is widely held to be the syntactic head of a subordinate clause, which is therefore often represented by the abbreviation CP (for complementizer phrase). Evidence that the complementizer functions as the head of its clause includes the fact that it is commonly the last element in a clause in languages like Korean or Japanese in which other heads follow their complements, and always first in "head-first" languages such as English.

It is common for the complementizers of a language to be borrowed from other syntactic categories (a process known as grammaticalization). Across the languages of the world, it is especially common for determiners to be used as complementizers (e.g. English that). Another frequent source of complementizer vocabulary is the class of interrogative words . It is especially common for a form that otherwise means what to be borrowed as a complementizer, but other interrogative words are often used as well; e.g. colloquial English I read in the paper how it's going to be cold today, with unstressed how roughly equivalent to that). English for in sentences like I would prefer for there to be a table in the corner shows a preposition borrowed as a complementizer. (The sequence for there in this sentence is clearly not a prepositional phrase.) In many languages of West Africa, the form of the complementizer can be related to the verb say.