Dual (abbreviated DU) is a grammatical number that some languages use in addition to singular and plural. When a noun or pronoun appears in dual form, it is interpreted as referring to precisely two of the entities (objects or persons) identified by the noun or pronoun acting as a single unit or in unison. Verbs can also have dual agreement forms in these languages.

The dual number existed in Proto-Indo-European and persisted in many of its descendants, such as Ancient Greek and Sanskrit, which have dual forms across nouns, verbs, and adjectives, Gothic, which used dual forms in pronouns and verbs, and Old English (Anglo-Saxon), which used dual forms in its pronouns. It can still be found in a few modern Indo-European languages such as Irish, Scottish Gaelic, Lithuanian, Slovene, and Sorbian languages.

The majority of modern Indo-European languages, including modern English, however, have lost dual through their development. Its function has mostly been replaced by the simple plural. They may hovewer show residual traces of dual, like for example in the English distinctions: both vs. all, either vs. any, neither vs. none, and so on. A commonly used sentence to exemplify dual in English is "Both go to the same school." where both refers to two specific people who had already been determined in the conversation.

Many Semitic languages also have dual numbers. For instance, in Hebrew יים‎- (-ayim) or a variation of it is added to the end of some nouns, e.g. some parts of the body (eye, ear, nostril, lip, hand, leg) and some time periods (minute, hour, day, week, month, year) to indicate that it is dual (regardless of how the plural is formed). A similar situation exists in classical Arabic, where ان -ān is added to the end of any noun to indicate that it is dual (regardless of how the plural is formed).

It is also present in those Khoisan languages that have a rich inflectional morphology, particularly Khoe languages, as well as Kunama, a Nilo-Saharan language.