Zayin (also spelled Zain or Zayn or simply Zay) is the seventh letter of many Semitic abjads, including Phoenician 𐤆, Aramaic, Hebrew ז, Syriac ܙ and Perso-Arabic alphabet ز (see below). It represents the sound [z].

The Phoenician letter gave rise to the Greek Zeta (Ζ), Etruscan z , Latin Z, and Cyrillic Ze З.

The Phoenician letter appears to be named after a sword or other weapon. (In Biblical Hebrew, "Zayin" (זין) means sword, and the verb "Lezayen" (לזיין) means to arm. In modern Hebrew, 'zayin' (זין) means penis and 'lezayen' (לזין) is a vulgar term which generally means to perform sexual intercourse and is used in a similar fashion to the English word fuck, although the older meaning survives in 'maavak mezuyan' (armed struggle) (מאבק מזוין) and 'beton mezuyan' (בטון מזוין) (armed, i.e., reinforced concrete). The Proto-Sinaitic glyph according to Brian Colless may have been called ziqq, based on a hieroglyph depicting a "manacle".

In modern Hebrew the frequency of the usage of zayin, out of all the letters, is 0.88%.