Tsade (also spelled ṣade, ṣādē, ṣaddi, ṣad, tzadi, sadhe, tzaddik) is the eighteenth letter of the Semitic abjads, including Phoenician ṣādē 𐤑, Hebrew ṣādī צ‎, Aramaic ṣāḏē 𐡑, Syriac ṣāḏē ܨ, Ge'ez ṣädäy ጸ, and Arabic ṣād ص‎. It is related to the Ancient North Arabian 𐪎‎‎, South Arabian 𐩮, and Ge'ez ጸ. The corresponding letter of the Ugaritic alphabet is 𐎕 ṣade.

Its oldest phonetic value is debated, although there is a variety of pronunciations in different modern Semitic languages and their dialects. It represents the coalescence of three Proto-Semitic "emphatic consonants" in Canaanite. Arabic, which kept the phonemes separate, introduced variants of ṣād and ṭāʾ to express the three (see ḍād, ẓāʾ). In Aramaic, these emphatic consonants coalesced instead with ʿayin and ṭēt, respectively, thus Hebrew ereṣ ארץ (earth) is araʿ ארע‎ in Aramaic.

The Phoenician letter is continued in the Greek san (Ϻ) and possibly sampi (Ϡ), and in Etruscan 𐌑 Ś. It may have inspired the form of the letter tse in the Glagolitic and Cyrillic alphabets.

The letter is named "tsadek" in Yiddish, and Hebrew speakers often give it a similar name as well. This name for the letter probably originated from a fast recitation of the alphabet (i.e., "tsadi, qoph" → "tsadiq, qoph"), influenced by the Hebrew word tzadik, meaning "righteous person".

The origin of ṣade is unclear. It may have come from a Proto-Sinaitic script based on a pictogram of a plant, perhaps a papyrus plant, or a fish hook (in Modern Hebrew, צד tsad means "[he] hunt[ed]", and in Arabic صاد ṣād means "[he] hunted"). The form of the Arabic letter ṣād may be formed from a ligature of dotless nūn and the bottom part of the letter ṭa.

The letter is named ṣād and in Modern Standard Arabic is pronounced /sˤ/.

It is written in several ways depending on its position in the word:

Chapter 38 of the Quran is named for this letter, which begins the chapter.