Fritz Zwicky (/ˈtsvɪki/; German: [ˈtsvɪki]; February 14, 1898 – February 8, 1974) was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and observational astronomy. He was the first to propose supernovas as giant explosions at the end of a star's life, and neutron stars as the remnants left over after supernovas. In 1933, Zwicky was the first to use the virial theorem to postulate the existence of unseen dark matter, describing it as "dunkle Materie".

Fritz Zwicky was born in Varna, Bulgaria, to a Swiss father (citizenship in Mollis, Glarus) and Czech mother. His father, Fridolin (b. 1868), was a prominent industrialist in the Bulgarian city and also served as Norwegian consul in Varna (1908–1933). Fridolin Zwicky designed and built his family's Zwicky House in Varna. Fritz's mother, Franziska Vrček (b. 1871), was an ethnic Czech of the Austro-Hungarian Empire. Fritz was the oldest of three children: he had a brother named Rudolf and a sister, Leonie. Fritz's mother died in Varna in 1927. His father lived and worked in Bulgaria until 1945, and returned to Switzerland after World War II. Fritz's sister Leonie married a Bulgarian from Varna and spent her entire life in the city.

In 1904, at the age of six, Fritz was sent to his paternal grandparents in Glarus, Switzerland, to study commerce. His interests shifted to math and physics. He received an advanced education in mathematics and experimental physics at the Swiss Federal Polytechnic (today known as ETH Zurich) in Zürich. He finished his studies there in 1922 with a Dr. sc. nat. degree (PhD equivalent) with a thesis entitled Zur Theorie der heteropolaren Kristalle (On the theory of heteropolar crystals).

In 1925, Zwicky emigrated to the United States to work with Robert Millikan at California Institute of Technology (Caltech) after receiving the Rockefeller Foundation fellowship. He had an office down the hall from Robert Oppenheimer.

Zwicky developed numerous cosmological theories that have had a profound influence on the understanding of our universe in the early 21st century. He coined the term "supernova" while fostering the concept of neutron stars. Five years passed before Oppenheimer published his landmark paper announcing "neutron stars".

Zwicky was appointed Professor of Astronomy at Caltech in 1942. He also worked as a research director/consultant for Aerojet Engineering Corporation (1943–1961), and as a staff member of Mount Wilson Observatory and Palomar Observatory for most of his career. He developed some of the earliest jet engines and holds more than 50 patents, many in jet propulsion. He invented the Underwater Jet. An example of his WWII rocket propulsion work would be a patent on a nitromethane engine filed by a collective of Zwicky and three other Aerojet employees in March 1944, and he also published an article on chemical kinetics in rocket engines and motors in 1950.

On 25 March 1932, Fritz Zwicky married Dorothy Vernon Gates (1904–1991), a member of a prominent local family and a daughter of California State Senator Egbert James Gates. Her money was instrumental in the funding of the Palomar Observatory during the Great Depression. Nicholas Roosevelt, cousin of President Theodore Roosevelt, was his brother-in-law by marriage to Tirzah Gates. Zwicky and Dorothy divorced amicably in 1941.

On 18 October 1947, Zwicky married Anna Margaritha Zürcher (1929–2012) in Switzerland. They had three daughters together, Margrit, Franziska, and Barbarina. The Zwicky Museum at the Landesbibliothek, Glarus, houses many of his papers and scientific works. Zwicky died in Pasadena, California on February 8, 1974, and was buried in Mollis, Switzerland.