The Codex Sinaiticus (Shelfmarks and references: London, British Library, Add MS 43725; Gregory-Aland nº א [Aleph] or 01, [Soden δ 2]), or "Sinai Bible", is one of the four great uncial codices, ancient, handwritten copies of a Christian Bible in Greek. The codex is a historical treasure.The codex is an Alexandrian text-type manuscript written in uncial letters on parchment and dated paleographically to the mid-4th century. Scholarship considers the Codex Sinaiticus to be one of the most important Greek texts of the New Testament, along with the Codex Vaticanus. Until Constantin von Tischendorf's discovery of the Sinaiticus text in 1844, the Codex Vaticanus was unrivaled.The Codex Sinaiticus came to the attention of scholars in the 19th century at Saint Catherine's Monastery in the Sinai Peninsula, with further material discovered in the 20th and 21st centuries. Although parts of the codex are scattered across four libraries around the world, most of the manuscript is held today in the British Library in London, where it is on public display. Since its discovery, study of the Codex Sinaiticus has proven to be useful to scholars for critical studies of biblical text.

While large portions of the Old Testament are missing, it is assumed that the codex originally contained the whole of both Testaments. About half of the Greek Old Testament (or Septuagint) survived, along with a complete New Testament, the entire Deuterocanonical books, the Epistle of Barnabas and portions of The Shepherd of Hermas.