Unknown was an American pulp fantasy fiction magazine, published from 1939 to 1943 by Street & Smith, and edited by John W. Campbell. Unknown was a companion to Street & Smith's science fiction pulp, Astounding Science Fiction, which was also edited by Campbell at the time; many authors and illustrators contributed to both magazines. The leading fantasy magazine in the 1930s was Weird Tales, which focused on shock and horror. Campbell wanted to publish a fantasy magazine with more finesse and humor than Weird Tales, and put his plans into action when Eric Frank Russell sent him the manuscript of his novel Sinister Barrier, about aliens who own the human race. Unknown's first issue appeared in March 1939; in addition to Sinister Barrier, it included H. L. Gold's "Trouble With Water", a humorous fantasy about a New Yorker who meets a water gnome. Gold's story was the first of many in Unknown to combine commonplace reality with the fantastic.

Pulp magazines were inexpensive fiction magazines that were published from 1896 until around 1955. The term "pulp" derives from the cheap wood pulp paper on which the magazines were printed. In contrast, magazines printed on higher-quality paper were called "glossies" or "slicks". The typical pulp magazine had 128 pages; it was 7 inches (18 cm) wide by 10 inches (25 cm) high, and 0.5 inches (1.3 cm) thick, with ragged, untrimmed edges. Pulps were the successors to the penny dreadfuls, dime novels, and short-fiction magazines of the 19th century.