Aleph (or alef or alif, transliterated ʾ) is the first letter of the Semitic abjads, including Phoenician ʾālep 𐤀‎, Hebrew ʾālef א‎, Aramaic ʾālap 𐡀‎, Syriac ʾālap̄ ܐ, Arabic ʾalif ا‎, and North Arabian 𐪑‎. It also appears as South Arabian 𐩱 and Ge'ez ʾälef አ.

These letters are believed to have derived from an Egyptian hieroglyph depicting an ox's head to describe the initial sound of *ʾalp, the West Semitic word for ox (compare Biblical Hebrew אֶלֶף ʾelef, 'ox'). The Phoenician variant gave rise to the Greek alpha (Α), being reinterpreted to express not the glottal consonant but the accompanying vowel, and hence the Latin A and Cyrillic А and possibly the Armenian letter Ա.

Phonetically, aleph originally represented the onset of a vowel at the glottis. In Semitic languages, this functions as a prosthetic weak consonant, allowing roots with only two true consonants to be conjugated in the manner of a standard three consonant Semitic root. In most Hebrew dialects as well as Syriac, the aleph is an absence of a true consonant, a glottal stop ([ʔ]), the sound found in the catch in uh-oh. In Arabic, the alif represents the glottal stop pronunciation when it is the initial letter of a word. In texts with diacritical marks, the pronunciation of an aleph as a consonant is rarely indicated by a special marking, hamza in Arabic and mappiq in Tiberian Hebrew. In later Semitic languages, aleph could sometimes function as a mater lectionis indicating the presence of a vowel elsewhere (usually long). When this practice began is the subject of some controversy, though it had become well established by the late stage of Old Aramaic (ca. 200 BCE). Aleph is often transliterated as U+02BE ʾ MODIFIER LETTER RIGHT HALF RING, based on the Greek spiritus lenis ʼ; for example, in the transliteration of the letter name itself, ʾāleph.