An agglutinative language is a language in which the words are formed by joining morphemes together. This term was introduced by Wilhelm von Humboldt in 1836 to classify languages from a morphological point of view. It was derived from the Latin verb agglutinare, which means "to glue together".

An agglutinative language is a form of synthetic language where each affix typically represents one unit of meaning (such as "diminutive", "past tense", "plural", etc.), and bound morphemes are expressed by affixes (and not by internal changes of the root of the word, or changes in stress or tone). Besides, and most importantly, in an agglutinative language affixes do not become fused with others, and do not change form conditioned by others.

Synthetic languages which are not agglutinative are called fusional languages; they sometimes combine affixes by "squeezing" them together, often changing them drastically in the process, and joining several meanings in one affix (for example, a single short verbal suffix means "past tense, perfect aspect, first person singular").

Agglutinative is sometimes used as a synonym for synthetic, although it technically is not. When used in this way, the word embraces fusional languages and inflected languages in general. It is also worth noting that the distinction between an agglutinative and a fusional language is often not a sharp one. Rather one should think of these as two ends of a continuum, with various languages falling more toward one end or the other. In fact, a synthetic language may present agglutinative features in its open lexicon but not in its case system: for example, German, Dutch and Esperanto.

Examples of agglutinative languages are Uralic languages, Altaic languages, Japanese, Korean, Dravidian languages, Inuktitut, Swahili, Malay, and some Mesoamerican languages including Nahuatl, Huastec, and Totonac. In the past, most of Iran and the Ancient Near East also spoke such languages, like Sumerian, Elamite, Hurrian, Urartian, Hattic, Gutian, Lullubi, Kassite.

Agglutinative languages are not entirely grouped by the family (although Finnish and Hungarian are related, as are possibly Japanese and Korean). It is possible that convergent evolution had many separate languages develop this property, but there seems to exist a preferred evolutionary direction from agglutinative synthetic languages to fusional synthetic languages, and then to non-synthetic languages, which in their turn evolve again into agglutinative synthetic languages.

Agglutinative languages tend to have a high rate of affixes/morphemes per word, and to be very regular. For example, Japanese has only three irregular verbs (and not very irregular), Nahuatl only two, and Turkish has only one.