Robert Anson Heinlein (July 7, 1907 – May 8, 1988) was an American novelist and science fiction writer. He is one of the most popular, influential, and controversial authors of "hard science fiction". He set a high standard for science and engineering plausibility and helped to raise the genre's standards of literary quality. He was the first writer to break into mainstream, general magazines such as The Saturday Evening Post, in the late 1940s, with unvarnished science fiction. He was among the first authors of bestselling, novel-length science fiction in the modern, mass-market era. For many years, Heinlein, Isaac Asimov, and Arthur C. Clarke were known as the "Big Three" of science fiction.

Within the framework of his science fiction stories Heinlein repeatedly integrated recognizable social themes: The importance of individual liberty and self-reliance, the obligation individuals owe to their societies, the influence of organized religion on culture and government, and the tendency of society to repress non-conformist thought. He also examined the relationship between physical and emotional love, speculated about unorthodox family relationships, and the influence of space travel on human cultural practices. His iconoclastic approach to these themes led to wildly divergent perceptions of his works and attempts to place mutually contradictory labels on his work. For example, his 1959 novel Starship Troopers was widely viewed as an advocacy of militarism and even to contain some elements of fascism, although many passages in the book disparage the inflexibility and stupidity of a purely militaristic mindset. By contrast, his 1961 novel Stranger in a Strange Land put him in the unexpected role of pied piper to the sexual revolution and the counterculture, and through this book he was credited with popularizing the notion of polyamory, or responsible nonmonogamy.

Heinlein won four Hugo Awards for his novels. In addition, fifty years after publication, three of his works were awarded "Retro Hugos" — awards given retrospectively for years in which no Hugos had been awarded. He also won the first Grand Master Award given by the Science Fiction Writers of America for lifetime achievement.

After his death, his wife Virginia Heinlein issued a compilation of Heinlein's correspondence and notes into a somewhat autobiographical examination of his career, published in 1989 under the title Grumbles from the Grave.

In his fiction, Heinlein coined words that have become part of the English language, including "grok", "TANSTAAFL" and "waldo."