Charles Ammi Cutter (March 14, 1837 – September 6, 1903) was an American librarian. In the 1850s and 1860s he assisted with the re-cataloging of the Harvard College library, producing America's first public card catalog. The card system proved more flexible for librarians and far more useful to patrons than the old method of entering titles in chronological order in large books. In 1868 he joined the Boston Athenaeum, making its card catalog an international model. Cutter promoted centralized cataloging of books, which became the standard practice at the Library of Congress. He was elected to leadership positions in numerous library organizations at the local and national level. Cutter is remembered for the Cutter Expansive Classification, his system of giving standardized classification numbers to each book, and arranging them on shelves by that number so that books on similar topics would be shelved together.

Cutter was born in Boston, Massachusetts. His aunt was an employee of the regional library in Boston. In 1856 Cutter was enrolled into Harvard Divinity School. He was appointed assistant librarian of the divinity school while still a student there and served in that capacity from 1857 to 1859. During that time, Cutter began designing a distinct cataloging schema for the library's outdated system. The catalog, dating from 1840, had a lack of order after the acquisition of 4,000 volumes from the collection of Professor Gottfried Christian Friedrich Lücke of the University of Göttingen, which added much depth to the Divinity School Library's collection.

During the 1857–1858 school year, Cutter rearranged the library collection on the shelves into broad subject categories along with classmate Charles Noyes Forbes. During the winter break of 1858–1859, they arranged the collection into a single listing alphabetically by author. This project was finished by the time Cutter graduated in 1859. By 1860 Cutter was already a seasoned staff member of the library and a full-time librarian. He became a journeyman to the chief cataloger and assistant librarian to Ezra Abbot. At Harvard College Cutter developed a new form of index catalog, using cards instead of published volumes, containing both an author index and a "classed catalog" or a rudimentary form of subject index.

In 1868 the Boston Athenæum library elected Cutter as its head librarian. His first assignment was to organize and aggregate the inventory of the library and develop a catalog from that and to publish a complete dictionary catalog for their collection. The previous librarian and assistants had been working on this, but much of the work was sub par and, according to Cutter, needed to be redone. This did not sit well with the trustees who wanted to get a catalog published as soon as possible. However, the catalog was revised and published in five volumes and is known as the Athenæum Catalogue. Cutter was the librarian at the Boston Athenaeum for twenty-five years (1869-1892).

In 1876, Cutter was hired by the United States Bureau of Education to help write a report about the state of libraries for the Centennial. Part two of this report was his Rules for a Printed Dictionary Catalogue (1876). This catalog was included in the organization's publication Public Libraries in the United States of America: Their History, Condition, and Management. Cutter implemented many ideologies familiar to contemporary librarians during his time at the Athenaeum. Cutter introduced characteristic structures and philosophies such as inter-library loan and furnishing every book with a pouch in the rear to encase a card in order to keep track of the item's circulating status.

Cutter served as editor of the Library Journal from 1891 to 1893. Of the many articles he wrote during this time, one of the most famous was an article called "The Buffalo Public Library in 1983". In it, he wrote what he thought a library would be like one hundred years in the future. He spent a lot of time discussing practicalities, such as how the library arranged adequate lighting and controlled moisture in the air to preserve the books.

In 1891 Cutter introduced an avant-garde and divergent system of cataloging he termed the Cutter Expansive Classification. This system incorporated seven levels of classification with the most basic libraries operating at the first level and the grandest, most distinguished institutions utilizing the seventh level, and it was Cutter's aspiration to orchestrate a classification system for every type of library. The classification system, or scheme, utilized an alpha-numeric methodology used to abbreviate authors' names and generate unique call numbers known as "Cutter numbers" or "Cutter codes". This scheme, on which the current Library of Congress cataloging system is partially based, in turn laid the foundation for the Library of Congress Subject Headings and the Sears List of Subject Headings. When Cutter began to delegate a new system for the library he initially chose the Dewey Decimal Classification, however he determined it was more beneficial to assign a more distinct adaptation for the collection. Even though Cutter's Expansive Classification was recognized as a significant contribution to libraries and to the burgeoning field of library science, Cutter himself did not champion its success nor did he anticipate future editions of his system. Nevertheless, his "Cutter numbers" survived and are still used in libraries today.

Cutter also devised a table for author numbers, meant to assist libraries assign book numbers. His table was revised by Kate Sanborn, his assistant at Boston Athenaeum and cataloger at St. Louis Mercantile Library. The three-figure tables are still used in contemporary library classification.