Al- (Arabic: ٱلْـ, also romanized as el-, il-, and l- as pronounced in some varieties of Arabic), is the definite article in the Arabic language: a particle (ḥarf) whose function is to render the noun on which it is prefixed definite. For example, the word كتاب kitāb "book" can be made definite by prefixing it with al-, resulting in الكتاب al-kitāb "the book". Consequently, al- is typically translated as "the" in English.

Unlike most other Arabic particles, al- is always prefixed to another word and never stands alone. Consequently, many dictionaries do not list it, and it is almost invariably ignored in collation, as it is not an intrinsic part of the word.

Al- does not inflect for gender, number or grammatical case. The sound of the final -l consonant, however, can vary; when followed by a sun letter such as t, d, r, s, n and a few others, it assimilates to that sound, thus doubling it. For example: for "the Nile", one does not say *al-Nīl, but an-Nīl. When followed by a moon letter, like m-, there is no assimilation: al-masjid ("the mosque"). This affects only the pronunciation and not the spelling of the article.

This article deals with the use of the definite article in Literary Arabic, which slightly differs among varieties of Arabic.

To put al- into perspective, there are many ways in which Arabic words can be made definite. These include the use of personal pronouns like "me", the use of proper nouns like "Saudi Arabia", demonstrative pronouns like "this man", relative pronouns like "the man who ...", vocation like "O man", possession like "my man", and of course the definite article like "the man". Apart from possession, prefixing a noun with al- is the weakest form of definiteness. That is, saying "the man" does not define the man being referred to as clearly as saying "this man", for example.

Arabic has an indefinite article indicated by nunation (tanwīn) which is declined for three cases.

There are several major opinions in regards to the origins of the Arabic definite article. The earliest evidence of the article, besides a 1st-century BC inscription in Qaryat al-Faw (formerly Qaryat Dhat Kahil, near Sulayyil, Saudi Arabia), occurs in the 5th century BC, in the epithet of a goddess which Herodotus (Histories I: 131, III: 8) quotes in its preclassical Arabic form as Alilat (Ἀλιλάτ, i. e.,ʼal-ʼilat), which means "the goddess".

While the Proto-Semitic language did not have any articles, the most likely theory is that the article al- comes from the same proto-Semitic source as the Hebrew definite article ה־ ha-. That theory is based primarily on the fact that the two share many similarities. Both particles are prefixed to nouns, and both geminate with certain following letters. Moreover, neither particle is prefixed to non-final nouns in a genitival construction. Finally, both are prefixed to relative clauses. According to David Testen, many northern and southwestern Semitic languages have particles that bear similarities to al-. With this fact, he posits that al- has a proto-Semitic antecedent.