A fanzine (blend of fan and magazine or zine) is a non-professional and non-official publication produced by enthusiasts of a particular cultural phenomenon (such as a literary or musical genre) for the pleasure of others who share their interest.

The term was coined in an October 1940 science-fiction fanzine by Russ Chauvenet and first popularized within science fiction fandom, and from there the term was adopted by other communities.

Typically, publishers, editors, writers and other contributors of articles or illustrations to fanzines are not paid. Fanzines are traditionally circulated free of charge, or for a nominal cost to defray postage or production expenses. Copies are often offered in exchange for similar publications, or for contributions of art, articles, or letters of comment (LoCs), which are then published.

Some fanzines are typed and photocopied by amateurs using standard home office equipment. A few fanzines have developed into professional publications (sometimes known as "prozines"), and many professional writers were first published in fanzines; some continue to contribute to them after establishing a professional reputation. The term fanzine is sometimes confused with "fan magazine", but the latter term most often refers to commercially produced publications for (rather than by) fans.

The origins of amateur fanac "fan" publications are obscure, but can be traced at least back to 19th century literary groups in the United States which formed amateur press associations to publish collections of amateur fiction, poetry, and commentary, such as H. P. Lovecraft's United Amateur.

As professional printing technology progressed, so did the technology of fanzines. Early fanzines were hand-drafted or typed on a manual typewriter and printed using primitive reproduction techniques (e.g., the spirit duplicator or even the hectograph). Only a very small number of copies could be made at a time, so circulation was extremely limited. The use of mimeograph machines enabled greater press runs, and the photocopier increased the speed and ease of publishing once more. Today, thanks to the advent of desktop publishing and self-publication, there is often little difference between the appearance of a fanzine and a professional magazine.

When Hugo Gernsback published the first science fiction magazine, Amazing Stories in 1926, he allowed for a large letter column which printed reader's addresses. By 1927 readers, often young adults, would write to each other, bypassing the magazine. Science fiction fanzines had their beginnings in Serious & Constructive (later shortened to sercon) correspondence. The fans would start up clubs to ease finding others with their same interests. Gernsback founded the Science Fiction League in 1934, where these clubs could advertise for more users.

The first science fiction fanzine, The Comet, was published in 1930 by the Science Correspondence Club in Chicago and edited by Raymond A. Palmer and Walter Dennis. The term "fanzine" was coined by Russ Chauvenet in the October 1940 edition of his fanzine Detours. "Fanzines" were distinguished from "prozines" (a term Chauvenet also invented), that is, all professional magazines. Prior to that, the fan publications were known as "fanmags".