Zeta (UK: /ˈziːtə/, US: /ˈzeɪtə/ ^ⓘ; uppercase Ζ, lowercase ζ; Ancient Greek: ζῆτα, Demotic Greek: ζήτα, classical [d͡zɛ̌ːta] or [zdɛ̌ːta] zē̂ta; Modern Greek: [ˈzita] zíta) is the sixth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 7. It was derived from the Phoenician letter zayin . Letters that arose from zeta include the Roman Z and Cyrillic З.

Unlike the other Greek letters, this letter did not take its name from the Phoenician letter from which it was derived; it was given a new name on the pattern of beta, eta and theta.

The word zeta is the ancestor of zed, the name of the Latin letter Z in Commonwealth English. Swedish and many Romance languages (such as Italian and Spanish) do not distinguish between the Greek and Roman forms of the letter; "zeta" is used to refer to the Roman letter Z as well as the Greek letter.

The letter ζ represents the voiced alveolar fricative IPA: [z] in Modern Greek.

The sound represented by zeta in Greek before 400 BC is disputed. See Ancient Greek phonology and Pronunciation of Ancient Greek in teaching.

Most handbooks^[who?] agree on attributing to it the pronunciation /zd/ (like Mazda), but some scholars believe that it was an affricate /dz/ (like adze). The modern pronunciation was, in all likelihood, established in the Hellenistic age and may have already been a common practice in Classical Attic; for example, it could count as one or two consonants metrically in Attic drama.^[where?]

Zeta has the numerical value 7 rather than 6 because the letter digamma (ϝ, also called 'stigma' as a Greek numeral) was originally in the sixth position in the alphabet.

The uppercase zeta is not used, because it is normally identical to Latin Z. The lower case letter can be used to represent: