Archbishop's Diocesan and Cathedral Library, Diocesan library in Cologne, Germany.

The Archbishop's Diocesan and Cathedral Library is a scholarly collection in Cologne that holds about 400,000 volumes and important medieval manuscripts. It focuses on Catholic theology and includes rare printed books from the 15th century.

The library traces back to the manuscript collection of Archbishop Hildebald (died 818); a catalogue from 833 lists 175 books, 35 of which survive. In 1794, 200 of the most precious manuscripts were taken to Arnsberg ahead of the French revolutionary army, and the cathedral chapter only recovered its library from the Prussian state in 1867. The diocesan library, re-founded in 1738, was built on the book collection of Johann Jakob von Broich, who had bought it from the estate of Abbé Jean-Paul Bignon, librarian to Louis XIV. Since 1930 the diocesan and cathedral libraries have been united under one name.

The library attracts theology students and scholars who study historical texts on Catholic faith and practice in its reading rooms. The place reflects the long bond between Cologne's church and the preservation of knowledge.

The library is located in a historic building and is open to researchers on several weekdays. Visitors should ask in advance whether registration is needed, especially if they want to view rare collections.

The CEEC project, run with the University of Cologne's historical-cultural information processing chair and funded by the German Research Foundation, digitized 394 medieval codices totalling 140,231 pages. This made the library the first in the world to put its entire medieval manuscript holdings online as a digital library.