William Isaac Fletcher (April 23, 1844 – June 15, 1917) was an American librarian, bibliographer, and indexer who served as the head librarian of Amherst College from 1883 to 1911 and the President of the American Library Association in 1891–92.

Born in Burlington, Vermont, Fletcher grew up in Winchester, Massachusetts.

After a decade and a half of working at several other libraries in Massachusetts and Connecticut, Fletcher was hired by Amherst in November 1883.

Fletcher served as the editor of four supplements (1893, 1897, 1903, and 1908) of the Poole's Index to Periodical Literature as well as of the 1893 and 1901 editions of the ALA Index to General Literature.

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