Zayin (also spelled zain or zayn or simply zay) is the seventh letter of the Semitic abjads, including Phoenician zayn 𐤆, Hebrew zayīn ז‎, Aramaic zain 𐡆, Syriac zayn ܙ, and Arabic zāy ز‎. It represents the sound [z]. It is also related to the Ancient North Arabian 𐪘‎‎, South Arabian 𐩸, and Ge'ez ዘ.‌ The Phoenician letter gave rise to the Greek zeta (Ζ), Etruscan z , Latin Z, and Cyrillic Ze З, as well as Ж.

The Proto-Sinaitic glyph may have been called ziqq, may not have been based on a hieroglyph, and may have depicted a "fetter".

An alternative view is that it is based on the "copper ingot" hieroglyph (𓈔) in the form of an axeblade, after noting that the name "zayin" has roots in Aramaic to refer to "Arms," "Armor," and "Metal used for arms."

The Phoenician letter appears to be named after a sword or other weapon. In Hebrew, zayin (זין‎) means "weapon", the verb lĕzayyēn (לזיין‎) means "to arm", and the verb lĕhizdayyēn (להזדיין‎) means "to arm oneself".

The letter is named zāy. It has two forms, depending on its position in the word:

The similarity to rāʼ  ر  is likely a function of the original Syriac forms converging to a single symbol, requiring that one of them be distinguished as a dot; a similar process occurred to jīm and ḥāʼ. In Maltese, the corresponding letter to ز is ż.

A variant letter of zāy named že is used in Persian with three dots above instead of just one dot above. The letter is used in a number of languages, such as Persian, Pashto, Kurdish, Urdu and Uyghur (see K̡ona Yezik̡).

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