A library classification is a system of coding and organizing library materials (books, serials, audiovisual materials, computer files, maps, manuscripts, realia) according to their subject. A classification consists of tables of subject headings and classification schedules used to assign a class number to each item being classified, based on that item's subject.

Until the 19th century, most libraries had closed stacks, so the library classification only served to organize the subject catalog. In the 20th century, libraries opened their stacks to the public and started to shelve the library material itself according to some library classification to simplify subject browsing.

There are many standard systems of library classification in use, and many more have been proposed over the years. These include the Library of Congress classification (LC) and the Dewey Decimal Classification (DDC), which are the most common systems in the English-speaking world. Other less commonly used classification systems include the Bliss bibliographic classification, the Colon classification and the Universal Decimal Classification. These systems are often considered to be theoretically superior, since they make use of the principle of synthesis (combining codes from different lists to represent the different attributes of a work), which is comparatively lacking in LC or DDC.

There are three main types of classification systems:

  • enumerative - produce an alphabetical list of subject headings, assign numbers to each heading in alphabetical order
  • hierarchial - divides subjects hierarchially, from most general to most specific
  • faceted or analytico-synthetic - divides subjects into mutually exclusive orthogonal facets

The most common classification systems, LC and DDC, are essentially enumerative, though with some hierarchial and faceted elements, especially at the broadest and most general level. The first true faceted system was the Colon classification of S. R. Ranganathan.

Specialist classification systems have been developed for particular subject areas, and some specialist libraries develop their own classification system that emphasises those areas they specialise in. An example specialist classification system for art and iconography is Iconclass.

Library classification forms part of the field of library and information science.