Day Keene was the pen name of American writer Gunard Hjertstedt (1904–1969), a novelist, short-story writer, and radio and television scriptwriter. Born in Chicago, he first worked as an actor in repertory theater before turning to writing. In the 1930s he contributed to pulp magazines and worked on the Little Orphan Annie radio program; another account also identifies him as head writer for Kitty Keene, Inc. His later career became closely associated with crime fiction.
Keene moved to Florida and wrote paperback originals, particularly fast-paced crime novels, for publishers including Gold Medal, Ace, Avon, and Graphic. His novels included stories adapted for film: Joy House appeared as a 1964 MGM film, and Chautauqua was released as The Trouble with Girls in 1969.