Harry Warner, Jr. (1922-2004) was a science fiction fan and historian. Warner became active in science fiction fandom in 1936, although he became more and more reclusive as time went on, earning the nickname the Hermit of Hagerstown by the 1950s.

Though he almost never attended conventions — which led to his earning the nickname "The Hermit of Hagerstown" — he became revered in the fandom community over many years for his letter-writing. It is estimated Warner wrote literally tens of thousands of letters to numerous science fiction fanzines and related publications within the genre, and his reputation became such that nearly every fanzine publisher in the country would send him a free copy of their zine as a matter of course. Without fail, Warner would dutifully read the zine and send a lengthy and thoughtful letter. Many fanzine editors have described getting their first letter from Warner as something like a rite of passage.

Warner wrote two book-length histories of fandom, considered classic nonfiction works in the field, as they are among the few historical studies of the field other than personal memoirs. They are All Our Yesterdays, covering the 1940s, and A Wealth of Fable, covering the 1950s. In the 1950s, Warner tried his hand at science fiction itself, publishing a few short stories in various magazines throughout the decade.

Beginning in 1939, he published Horizons, and later published a two volume history of fandom in the 40s and 50s, All Our Yesterdays and A Wealth of Fable. The latter book won Warner a Hugo Award in 1993 for Best Related Book. He also won two Hugos for Best Fan Writer. In 1995, Warner received the First Fandom Hall of Fame Award. He remained active in fanzine fandom, which did not require travel, until the end of his life.

Such was Warner's erudition that he is known to have taught himself no fewer than seven languages.

Warner, who spent most of his professional life working for the Hagerstown Herald-Mail, died February 17, 2003 at the age of 80.