Codex Sinaiticus (London, Brit. Libr., Add. 43725; Gregory-Aland no. א or 01) is a complete, 4th century uncial manuscript of the New Testament. It also contains substantial portions of the Septuagint. Along with Codex Vaticanus, Codex Sinaticus is one of the most valuable manuscripts of the Greek New Testament.

Codex Sinaiticus that was found by Constantin von Tischendorf on his third visit to the convent of Saint Catherine, on Mount Sinai in Egypt, in 1859. The first two trips had yielded parts of the Old Testament, some from a rubbish bin. The emperor Alexander II of Russia sent him to search for manuscripts, which he was convinced were still to be found in the Sinai convent.

The story of his finding the manuscript, which contained most of the Old Testament and all of the New Testament, has all the interest of a romance. He reached the convent on January 31; but his inquiries appeared to be fruitless. On the 4th February he had resolved to return home without having gained his object. "On that day, when walking with the provisor of the convent, he spoke with much regret of his ill-success. Returning from their promenade, Tischendorf accompanied the monk to his room, and there had displayed to him what his companion called a copy of the Septuagint, which he, the ghostly brother, owned. The MS. was wrapped up in a piece of cloth, and on its being unrolled, to the surprise and delight of the critic the very document presented itself which he had given up all hope of seeing. His object had been to complete the fragmentary Septuagint of 1844, which he had declared to be the most ancient of all Greek codices on vellum that are extant; but he found not only that, but a copy of the Greek New Testament attached, of the same age, and perfectly complete, not wanting a single page or paragraph." This precious fragment, after some negotiations, he obtained possession of, and conveyed it to the Emperor Alexander, who fully appreciated its importance, and caused it to be published as nearly as possible in facsimile, so as to exhibit correctly the ancient handwriting.

The entire codex consists of 346 1/2 folios, written in four columns. Of these 199 belong to the Old Testament and 147 1/2 to the New, along with two other books, the Epistle of Barnabas and part of The Shepherd of Hermas. The books of the New Testament are arranged in this order: the four Gospels, the epistles of Paul, the Acts of the Apostles, the Catholic Epistles, Revelation. Later parts of Genesis and Numbers were found binding other books and sent to Tischendorf.

Of its prior history, little is known. The colophons to Esdras and Esther indicated that it had been in Caesarea in the 6th or 7th centuries. It is speculated to have been written in Egypt.

Codex Sinaticus was purchased by the British Library in 1933 from the Soviet Union for £100,000.