The Codex Calixtinus (or Codex Compostellus) is a manuscript that is the main witness for the 12th-century Liber Sancti Jacobi ('Book of Saint James'), a pseudepigraph attributed to Pope Calixtus II. The principal author or compiler of the Liber is thus referred to as "Pseudo-Calixtus", but is often identified with the French scholar Aymeric Picaud. Its most likely period of compilation is 1138–1145.

It was intended as an anthology of background detail and advice for pilgrims following the Way of Saint James to the shrine of the apostle Saint James the Great, located in the cathedral of Santiago de Compostela, Galicia. The collection includes sermons, reports of miracles and liturgical texts associated with Saint James, and a set of polyphonic musical pieces. In it are also found descriptions of the route, works of art to be seen along the way, and the customs of the local people.

The compilation of Codex Calixtinus predates 1173, most likely taking place during the late 1130s to early 1140s. This compilation is most likely due to the French scholar Aymeric Picaud. Each of the five books is prefaced with a pseudepigraphic letter attributed to Pope Calixtus II (d. 1124). The appendix contains a letter by Pope Innocent II (d. 1143), presenting the finished work to Santiago. There are some clues suggestive of a later date of around 1160, but none of them render impossible a date of around 1140. The miracles in book II are recounted with their dates, between 1080 and 1135, so that the completion of the compilation can with some certainty be dated to between 1135 and 1173, and with highest probability to the 1140s.

While the individual texts have a complex history, and each of the five books was probably in existence before their compilation in a single "encyclopedia for the pilgrimage and cult of St. James", Codex Calixtinus is the archetype manuscript for the composite Liber sancti Jacobi. For this reason, the terms Liber sancti Jacobi and Codex Calixtinus are often used interchangeably.

The historical content of the compilation is emergence of Saint James as a patron saint for the fight against Islam in Iberia. It has also been suggested that the book was written in deliberately bad Latin and is actually a kind of grammar book.

The oldest copy of the Codex, known as The Ripoll (after the monastery of Santa Maria de Ripoll in Catalonia) was made in 1173 by the monk Arnaldo de Monte. This date serves as terminus ante quem for the compilation of the Liber (excluding appendices). Many later copies of the work exist.

Codex Calixtinus was long held in the archives of the Cathedral of Santiago de Compostela and was rediscovered there by the Jesuit scholar Padre Fidel Fita in 1886. The first edition of the text was prepared in 1932 by Walter Muir Whitehill, and published in 1944 by Spain's Consejo Superior de Investigaciones Científicas, together with a musicological study by Silos's Dom Germán Prado O.S.B., and another on the miniature illustrations by Jesús Carro García.

The book was stolen from its security case in the cathedral's archives on 3 July 2011. Spanish press reports speculated that the theft may have been an attempt to embarrass the cathedral administration over lax security measures or an attempt to settle a personal or professional grievance.