The Cairo Geniza, alternatively spelled the Cairo Genizah, is a collection of some 400,000 Jewish manuscript fragments and Fatimid administrative documents that were kept in the genizah or storeroom of the Ben Ezra Synagogue in Fustat or Old Cairo, Egypt. These manuscripts span the entire period of Middle-Eastern, North African, and Andalusian Jewish history between the 6th and 19th centuries CE, and comprise the largest and most diverse collection of medieval manuscripts in the world.

The Genizah texts are written in various languages, especially Hebrew, Arabic, and Aramaic, mainly on vellum and paper, but also on papyrus and cloth. In addition to containing Jewish religious texts such as Biblical, Talmudic, and later Rabbinic works (some in the original hands of the authors), the Genizah gives a detailed picture of the economic and cultural life of the Mediterranean region, especially during the 10th to 13th centuries.

Manuscripts from the Cairo Geniza are now dispersed among a number of libraries, including the Cambridge University Library, the Jewish Theological Seminary of America, the John Rylands Library, the Bodleian Library, the University of Pennsylvania's Katz Center for Advanced Judaic Studies, the British Library, the Hungarian Academy of Sciences, the National Library of Russia, Alliance Israélite Universelle, the Younes and Soraya Nazarian Library at the University of Haifa and multiple private collections around the world. Most fragments come from the geniza chamber of the Ben Ezra Synagogue, but additional fragments were found at excavation sites near the synagogue and in the Basatin cemetery east of Old Cairo. Modern Cairo Geniza manuscript collections include some old documents that collectors bought in Egypt in the latter half of the nineteenth century.

In the 1890s, the geniza was a window- and doorless room, only accessible from the Ben Ezra Synagogue galleries by climbing a ladder to an opening up in the wall. Relatively large, it was filled with manuscripts and printed books thrown in through the opening over the centuries, mangled and sometimes glued to each other, covered in dust and infested with insects. By the late 19th century, some papes had been stolen, most likely by the synagogue beadles, and sold as souvenirs to tourists.

The first European to note the collection was apparently Simon van Gelderen (a great-uncle of Heinrich Heine), who visited the Ben Ezra synagogue and reported about the Cairo Genizah in 1752 or 1753. In 1864, the traveler and scholar Jacob Saphir visited the synagogue and explored the Genizah for two days; while he did not identify any specific item of significance he suggested that possibly valuable items might be in store. After a major synagogue renovation, Jewish book collector Elkan Nathan Adler was the first West European to enter the geniza in 1896, when he purchased a sackful of documents, which however failed to impress the researchers back in England.

In 1896, the Scottish scholars and twin sisters Agnes S. Lewis and Margaret D. Gibson returned from Egypt with fragments from the Genizah they considered to be of interest, and showed them to Solomon Schechter, "their irrepressibly curious rabbinical friend" at Cambridge. Schechter, already aware of the Genizah but not of its significance, immediately recognized the importance of the material. With the financial assistance of his Cambridge colleague and friend Charles Taylor, Schechter made an expedition to Egypt, where, with the assistance of the Chief Rabbi, he sorted and removed the greater part of the contents of the Genizah chamber. Agnes and Margaret joined him there en route to Sinai (their fourth visit in five years) and he showed them the chamber which Agnes reported was "simply indescribable".

Egyptologist Count Riamo d'Hulst discovered that Schechter, who had worked alone, had had to limit himself to the already herculean task of going through the material inside the geniza, and had neither fully finished that enterprise, nor had he dealt with the huge volume of written material stored in the open in the synagogue courtyard during the 1889-1892 renovation of the synagogue. That material had in part also originated from the geniza. Count d'Hulst wrote that much of the courtyard was covered one metre high with documents. Much of this material had been left outside after the renovation, and d'Hulst led an almost two months long excavation of the grounds. By May 1898, he was able to send to the Bodleian Library 16 large sacks of documents.

The Genizah fragments have now been archived in various libraries around the world. The Taylor-Schechter collection at Cambridge is the largest, by far, single collection, with nearly 193,000 fragments (137,000 shelf-marks). There are a further 43,000 fragments at the Jewish Theological Seminary Library. The John Rylands University Library in Manchester holds a collection of over 11,000 fragments, which are currently being digitised and uploaded to an online archive. The Bodleian Library at the University of Oxford has a collection of 25,000 Genizah folios.