Shin (also spelled Šin (šīn) or Sheen) is the twenty-first and penultimate letter of the Semitic abjads, including Phoenician šīn 𐤔, Hebrew šīn ש‎, Aramaic šīn 𐡔, Syriac šīn ܫ, and Arabic sīn س‎.

The Phoenician letter gave rise to the Greek Sigma (Σ) (which in turn gave rise to the Latin S, the German ẞ and the Cyrillic С), and the letter Sha in the Glagolitic and Cyrillic scripts (, Ш). The South Arabian and Ethiopian letter Śawt is also cognate. The letter šīn is the only letter of the Arabic alphabet with three dots with a letter corresponding to a letter in the Northwest Semitic abjad or the Phoenician alphabet.

The Proto-Sinaitic glyph, according to William Albright, was based on a "tooth" and with the phonemic value š "corresponds etymologically (in part, at least) to original Semitic ṯ (th), which was pronounced s in South Canaanite". However, the Proto-Semitic word for "tooth" has been reconstructed as *šinn-.

The Phoenician šin letter expressed the continuants of two Proto-Semitic phonemes, and may have been based on a pictogram of a tooth (in modern Hebrew shen).

The history of the letters expressing sibilants in the various Semitic alphabets is somewhat complicated, due to different mergers between Proto-Semitic phonemes. As usually reconstructed, there are nine Proto-Semitic coronal fricative phonemes that evolved into the various sibilants of its daughter languages, as follows:

Based on Semitic linguists (hypothesized), Samekh has no surviving descendant in the Arabic alphabet, and that sīn is derived from Phoenician šīn 𐤔 rather than Phonecian sāmek 𐤎, but it corresponds exclusively to Arabic س Sīn when comparing etymologically to other Semitic languages. In the Mashriqi abjadi order س sīn takes the place of Samekh at 15th position; meanwhile, the ش shīn is placed at the 21st position, represents /ʃ/, and is the 13th letter of the modern hijā’ī (هِجَائِي) or alifbāʾī (أَلِفْبَائِي) order and is written thus:

In the Arabic alphabet, according to McDonald (1986), "there can be no doubt that ش‎‎ is a formal derivative of س and that س is descended from 𐡔." but unlike the Hebrew ש‎ Sīn/Šīn and Aramaic 𐡔‎‎ Sīn/Šīn, Arabic س Sīn is considered a completely separate letter from ش Šīn /ʃ/.

The Arabic letter shīn was an acronym for "something" (شيء shayʾ(un) [ʃajʔ(un)]) meaning the unknown in algebraic equations. In the transcription into Spanish, the Greek letter chi (χ) was used which was later transcribed into Latin x. The letter shīn, along with Ṯāʾ, are the only two surviving letters in Arabic with three dots above. According to some sources, this is the origin of x used for the unknown in the equations. However, according to other sources, there is no historical evidence for this. In Modern Arabic mathematical notation, س sīn, i.e. shīn without its dots, often corresponds to Latin x. This led a debate to many Semitic linguists that the letter shīn is Arabic for samekh, although many Semitic linguists argue this debate as samekh has no surviving descendant in the Arabic alphabet.