John Whiteside Parsons (born Marvel Whiteside Parsons; October 2, 1914 – June 17, 1952) was an American rocket engineer, chemist, and Thelemite occultist. Parsons was one of the principal founders of both the Jet Propulsion Laboratory (JPL) and Aerojet. He invented the first rocket engine to use a castable, composite rocket propellant, and pioneered the advancement of both liquid-fuel and solid-fuel rockets.

Parsons was raised in Pasadena, California. He began amateur rocket experiments with school friend Edward Forman in 1928. Parsons was admitted to Stanford University but left before graduating due to financial hardship during the Great Depression. In 1934, Parsons, Forman, and Frank Malina formed the Caltech-affiliated Guggenheim Aeronautical Laboratory (GALCIT) Rocket Research Group, with support by GALCIT chairman Theodore von Kármán. The group worked on Jet-Assisted Take Off (JATO) for the U.S. military, and founded Aerojet in 1942 to develop and sell JATO technology during World War II. The GALCIT Rocket Research Group became JPL in 1943.

In 1939, Parsons converted to Thelema, a religious movement founded by English occultist Aleister Crowley. Parsons and his first wife, Helen Northrup, joined Crowley's Ordo Templi Orientis (O.T.O.); he became the California O.T.O. branch leader in 1942. Historians of Western esotericism cite him as a prominent figure in propagating Thelema in North America. Parsons was dismissed from JPL and Aerojet in 1944, due to his involvement with O.T.O. and his hazardous laboratory practices. In 1945, he and Helen divorced. In 1946, he married Marjorie Cameron. Shortly afterward, L. Ron Hubbard defrauded Parsons of his life savings.

Parsons worked as an explosives expert during the late 1940s, but his career in rocketry ended due to accusations of espionage and the increasing trend of McCarthyism. Parsons died at the age of 37 in a home laboratory explosion in 1952; his death was officially ruled an accident but many of his associates suspected suicide or murder. Although publicly unknown during his lifetime, Parsons is now recognized for his innovations in rocket engineering, advocacy of space exploration and human spaceflight, and as an important figure in the history of the U.S. space program. He has been the subject of several biographies and fictionalized portrayals.

Marvel Whiteside Parsons was born on October 2, 1914, at the Good Samaritan Hospital in Los Angeles. His parents, Ruth Virginia Whiteside (c. 1893–1952) and Marvel H. Parsons (c. 1894–1947), had moved to California from Massachusetts the previous year, purchasing a house on Scarff Street in downtown Los Angeles. Their son was his father's namesake, but was known in the household as Jack. The marriage broke down soon after Jack's birth, when Ruth discovered that her husband was sexually involved with a prostitute. Ruth filed for divorce in March 1915. Jack's father returned to Massachusetts after being exposed as an adulterer, with Ruth forbidding him from having any contact with his infant son. Marvel Parsons later joined the U.S. Armed Forces, reaching the rank of major, and married a woman with whom he had a son, Charles, a half-brother Jack only met once. Although she retained her ex-husband's surname, Ruth started calling her son John, but many friends throughout his life knew him as Jack.

Ruth's parents—Walter and Carrie Whiteside—moved to California to be with Jack and their daughter, purchasing an upscale house on Orange Grove Boulevard in Pasadena—known locally as "Millionaire's Mile"—where they could live together. Jack was surrounded by domestic servants. Having few friends, he lived a solitary childhood and spent much time reading; he took a particular interest in works of mythology, Arthurian legend, and the Arabian Nights. Through the works of Jules Verne he became interested in science fiction and a keen reader of pulp magazines like Amazing Stories, which led to his early interest in rocketry.

At age 12, Parsons began attending Washington Junior High School, where he performed poorly—which biographer George Pendle attributed to undiagnosed dyslexia—and was bullied for his upper-class status and perceived effeminacy. Although unpopular, he formed a strong friendship with Edward Forman, a boy from a poor working-class family who defended him from bullies and shared his interest in science fiction and rocketry. In 1928 the pair—adopting the Latin motto per aspera ad astra (through hardship to the stars)—began engaging in homemade gunpowder-based rocket experiments in the nearby Arroyo Seco canyon, as well as the Parsons family's back garden, which left it pockmarked with craters from explosive test failures. They incorporated commonly available fireworks such as cherry bombs into their rockets, and Parsons suggested using glue as a binding agent to increase the rocket fuel's stability. This research became more complex when they began using materials such as aluminium foil to make the gunpowder easier to cast. Parsons had also begun to investigate occultism, and performed a ritual intended to invoke the Devil into his bedroom; he worried that the invocation was successful and was frightened into ceasing these activities. In 1929, he began attending John Muir High School, where he maintained an insular friendship with Forman and was a keen participant in fencing and archery. After he received poor school results, Parsons's mother sent him away to study at the Brown Military Academy for Boys, a private boarding school in San Diego, but he was expelled for blowing up the toilets.

The Parsons family spent mid-1929 touring Europe before returning to Pasadena, where they moved into a house on San Rafael Avenue. With the onset of the Great Depression their fortune began to dwindle, and in July 1931 Jack's grandfather Walter died. Parsons began studying at the privately run University School, a liberal institution that took an unconventional approach to teaching. He flourished academically, becoming editor of the school newspaper, El Universitano, and winning an award for literary excellence; teachers who had trained at the nearby California Institute of Technology (Caltech) guided his attention to the study of chemistry. With the family's financial difficulties deepening, Parsons began working on weekends and school holidays at the Hercules Powder Company, where he learned more about explosives and their potential use in rocket propulsion. He and Forman continued to independently explore the subject in their spare time, building and testing different rockets, sometimes with materials that Parsons had stolen from work. Parsons soon constructed a solid-fuel rocket engine.