History of U.S. science fiction and fantasy magazines to 1950
Science-fiction and fantasy magazines began to be published in the United States in the 1920s. Stories with science-fiction themes had been appearing for decades in pulp magazines such as Argosy, but there were no magazines that specialized in a single genre until 1915, when Street & Smith, one of the major pulp publishers, brought out Detective Story Magazine.
Source: Wikipedia — History of U.S. science fiction and fantasy magazines to 1950 (CC BY-SA 4.0)