The Catholic Encyclopedia: An International Work of Reference on the Constitution, Doctrine, Discipline, and History of the Catholic Church, also referred to as the Old Catholic Encyclopedia and the Original Catholic Encyclopedia, is an English-language encyclopedia about Catholicism published in the United States. It was designed "to give its readers full and authoritative information on the entire cycle of Catholic interests, action and doctrine".

The first volume of the Catholic Encyclopedia appeared in March 1907 and the last three volumes appeared in 1912, followed by a master index volume in 1914 and later supplementary volumes. Its successor, the New Catholic Encyclopedia, was first published by the Catholic University of America in 1967.

The Catholic Encyclopedia was published by the Robert Appleton Company (RAC) in New York City. RAC was a publishing company incorporated in February 1905 for the express purpose of publishing the encyclopedia. The five members of the encyclopedia's editorial board also served as the RAC directors. In 1912, the company changed its name was changed to The Encyclopedia Press. Publication of the encyclopedia's volumes was the sole business conducted by Encyclopedia Press during the project's lifetime.

The Catholic Encyclopedia was designed to serve Catholics and other readers who wanted information about the church from the Catholic point of view. The encyclopedia records the accomplishments of Catholics and others in nearly all intellectual and professional pursuits, including the arts, education and science. While more limited in focus than other general encyclopedias, it was far broader in scope than previous efforts at comprehensive Catholic encyclopedias, which covered only internal Church affairs.

The Catholic Encyclopedia offers in-depth portrayals of historical and philosophical ideas, persons and events, from a Catholic perspective, including issues that divide Catholicism from Protestantism and other faith communities. Since the encyclopedia was first published starting in 1907 and has never been updated (versus the New Catholic Encyclopedia), many of its entries may be out of date either with respect to the wider culture or to the Catholic ecclesiastical world. In particular, it predates the creation of the Vatican City State (1929) and the Second Vatican Council (1962–1965), which introduced changes to Catholic practice.

The writing of the Catholic Encyclopedia began in 1905, under the supervision of five editors:

RAC published the first edition of the Catholic Encyclopedia. It released the volumes sequentially, the first two in 1907 and the last three in 1912:

The five editors of the encyclopedia convened their first editorial meeting at the offices of the Messenger in Manhattan. In addition to frequent informal conferences and constant communication by letters, the editors held 134 formal meetings to consider the plan, scope and progress of the multi-volume reference work This review process may have been accelerated by the reuse of older authorized publications. The editors received a nihil obstat, a declaration of no objection, from Remy Lafort, an official church censor, on November 1, 1908. Archbishop John Farley of New York then gave the first volumes of the encyclopedia his imprimatur.