Robert Albert Bloch (/blɒk/; April 5, 1917 – September 23, 1994) was an American fiction writer, primarily of crime, psychological horror, and fantasy, much of which has been dramatized for radio, cinema and television. He also wrote a relatively small amount of science fiction. His writing career lasted 60 years, including more than 30 years in television and film. He began his professional writing career immediately after graduation from high school, aged 17. He is best known as the writer of the novel Psycho (1959), the basis for the 1960 film Psycho directed by Alfred Hitchcock. Bloch wrote hundreds of short stories and over 30 novels. He was a protégé of H. P. Lovecraft, who was the first to seriously encourage his talent. However, while he started emulating Lovecraft and his brand of cosmic horror, he later specialized in crime and horror stories, often emphasizing psychological aspects of the characters within.

Bloch was a contributor to pulp magazines such as Weird Tales in his early career, and was also a prolific screenwriter and a major contributor to science fiction fanzines and fandom in general.

He won the Hugo Award (for his story "That Hell-Bound Train"), the Bram Stoker Award, and the World Fantasy Award. He served a term as president of the Mystery Writers of America (1970) and was a member of that organization and of Science Fiction Writers of America, the Writers Guild of America, the Academy of Motion Picture Arts and Sciences and the Count Dracula Society. In 2008, The Library of America selected Bloch's essay "The Shambles of Ed Gein" (1962) for inclusion in its two-century retrospective of American true crime.

His favorites among his own novels were The Kidnapper, The Star Stalker, Psycho, Night-World, and Strange Eons. His work has been extensively adapted into films, television productions, comics, and audiobooks.

Bloch was born in Chicago, the son of Raphael "Ray" Bloch (1884–1952), a bank cashier, and his wife Stella Loeb (1880–1944), a social worker, both of German Jewish descent. Bloch's family moved to Maywood, a Chicago suburb, when he was five; he lived there until he was ten. He attended the Methodist Church there, despite his parents' Jewish heritage, and studied at Emerson Grammar School. In 1925, at eight years of age, living in Maywood, he attended (alone at night) a screening of Lon Chaney, Sr.'s film The Phantom of the Opera (1925). The scene of Chaney removing his mask terrified the young Bloch ("it scared the living hell out of me and I ran all the way home to enjoy the first of about two years of recurrent nightmares"). It also sparked his interest in horror. Bloch was a precocious child and found himself in fourth grade when he was eight. He also obtained a pass into the adult section of the public library, where he read omnivorously. Bloch considered himself a budding artist and worked in pencil sketching and watercolours, but myopia in adolescence seemed to effectively bar art as a career. He had passions for German-made lead toy soldiers and for silent cinema.

In 1929, Bloch's father Ray Bloch lost his bank job, and the family moved to Milwaukee, where Stella worked at the Milwaukee Jewish Settlement settlement house. Robert attended Washington, then Lincoln High School, where he met lifelong friend Harold Gauer. Gauer was editor of The Quill, Lincoln's literary magazine, and accepted Bloch's first published short story, a horror story titled "The Thing" (the "thing" of the title was Death). Both Bloch and Gauer graduated from Lincoln in 1934 during the height of the Great Depression. Bloch was involved in the drama department at Lincoln and wrote and performed in school vaudeville skits.

During the 1930s, Bloch was an avid reader of the pulp magazine Weird Tales, which he had discovered at the age of ten in 1927. In the Chicago Northwestern Railroad depot with his parents and aunt Lil, his aunt offered to buy him any magazine he wanted and he picked Weird Tales (Aug 1927 issue) off the newsstand over her shocked protest. He began his readings of the magazine with the first instalment of Otis Adelbert Kline's "The Bride of Osiris" which dealt with a secret Egyptian city called Karneter located beneath Bloch's birth city of Chicago. The Depression came in the early 1930s. He later recalled, in accepting the Lifetime Achievement Award at the First World Fantasy Convention (1975), how "times were very hard. Weird Tales cost twenty-five cents in a day when most pulp magazines cost a dime. I remember that meant a lot to me." He went on to relate how he would get up very early on the last day of the month, with twenty-five cents saved from his monthly allowance of one dollar, and would run all the way to a combination tobacco/magazine store and buy the new Weird Tales issue, sometimes smuggling it home under his coat if the cover was particularly risqué. His parents were not impressed with Hugh Doak Rankin's sexy covers for the magazine, and when the Bloch family moved to Milwaukee in 1928 young Bloch gradually abandoned his interest. But by the time he had entered high school, he returned to reading Weird Tales during convalescence from flu.

H. P. Lovecraft, a frequent contributor to Weird Tales, became one of his favorite writers. The first of Lovecraft's stories he had read was "Pickman's Model", in Weird Tales for October 1927. Bloch wrote: "In school I was forced to squirm my way through the works of Oliver Wendell Holmes, James Lowell and Henry Wadsworth Longfellow. In 'Pickman's Model', the ghouls ate all three. Now that, I decided, was poetic justice." As a teenager, Bloch wrote a fan letter to Lovecraft (1933), asking where he could find copies of earlier stories of Lovecraft's that Bloch had missed. Lovecraft lent them to him. Lovecraft also gave Bloch advice on his early fiction-writing efforts, asking whether Bloch had written any weird work and, if so, whether he might see samples of it. Bloch took up Lovecraft's offer in late April 1933, sending him two short items, "The Gallows" and another work whose title is unknown.