Preston Ercelle Cloud, Jr. (September 26, 1912 – January 16, 1991) was an American earth scientist, biogeologist, cosmologist, and paleontologist. He served in the United States Navy (in which he was a bantamweight boxing champion), and led several field explorations of the U.S. Geological Survey. In academia, he was a member of the faculty of Harvard University, University of Minnesota, University of California, Los Angeles, and lastly University of California, Santa Barbara. He was best known for his work on the geologic time scale and the origin of life on Earth, and as a pioneering ecologist and environmentalist. His works on the significance of Cambrian fossils in the 1940s led to the development of the concept "Cambrian explosion," for which he coined the phrase "eruptive evolution."

Cloud was born in West Upton, Massachusetts. He was the third of seven children of Preston Ercelle and Pauline L. (née Wiedemann) Cloud. His father was an engineer-draftsman, and his mother, a genealogist. He grew up in Waynesboro, Pennsylvania, where he developed a love for the outdoors. He enjoyed Scouting and was an Eagle Scout. Upon graduating from Waynesboro High School in 1929, Cloud spent three years in the United States Navy (1930–1933) where he excelled at boxing. He was the bantamweight champion of the Pacific Fleet Scouting Force. Due to the Great Depression, he could not join university, and attended night school at George Washington University, so that he could self-support by doing odd jobs during daytime. Ray S. Bassler, his geology professor and Curator of Geology at the U.S. National Museum, helped him to get manual jobs in the museum. Because of his keen interest in paleontology, he was given a job as a preparator in the paleontology laboratory under G. Arthur Cooper. It was Cooper who guided him to have his first technical publication on Devonian brachiopods from Illinois in 1938. In 1937 he earned his BSc degree, and was elected to Phi Beta Kappa. He enrolled for PhD at Yale University graduate school in geology the same year. Working on the major systematic monograph on a group of Paleozoic brachiopods, under the supervision of C. O. Dunbar, he received his doctorate in 1940. His thesis directly earned him the A. Cressey Morrison Prize in Natural History of the New York Academy of Sciences, and was published by the Geological Society of America as Special Paper 38 in 1942.

In the summer of 1939, Cloud worked as field assistant to A. Lincoln Washburn on Victoria Island in western Arctic Canada. For the rest of the year he taught at the Missouri School of Mines in Rolla. During 1941 to 1942 he returned to Yale University to continue work on brachiopod evolution as a Sterling Research Fellow. At the height of World War II, he was recruited to the United States Geological Survey to work on the wartime Strategic Minerals Program. He embarked on a field party to investigate manganese deposits in Maine during the summer of 1941. In 1943, he joined Virgil Barnes of the Texas Bureau of Economic Geology in the Ellenburger Project, studying the stratigraphy and sedimentology of this important early Paleozoic carbonate complex. He became chief of party for bauxite investigations in Alabama.

After the war, in 1946, Cloud was appointed as Assistant Professor of Paleontology and Curator of Invertebrate Paleontology at Harvard University. In 1948 he returned to the U.S. Geological Survey to become chief of the Branch of Paleontology and Stratigraphy. He was chief of party to map and investigate the geology of Saipan in the Mariana Islands in the western Pacific. He became Chief Paleontologist from 1949 to 1959. In 1961 he was appointed as Chairman of the Department of Geology and Geophysics at the University of Minnesota. He established the School of Earth Sciences at that university, becoming its first Head. In 1965 he moved to the University of California, Los Angeles, as Professor of Biogeology, jointly with the Institute of Geophysics and the Department of Geology. In 1968 he transferred to the University of California, Santa Barbara campus, as full Professor. At UCSB he founded the Preston Cloud Research Laboratory, originally dedicated to paleomicrobiology and to studies of the first lunar geological samples from the Apollo 11 space mission. In addition, between 1974 and 1979 he was again a member of the U.S. Geological Survey based in Santa Barbara. He retired in 1974, but continued as Professor Emeritus. He spent the rest of his life at UCSB.

Cloud was a member of the National Academy of Sciences for thirty years, he was chairman of the Geology Section and occupied positions in its Council and Executive Committee. In 1967-69 he headed the Academy's "Committee on Resources and Man," whose alarming report "Resources and Man" introduced to a wider public, among other things, the work on energy-resources of his friend M. King Hubbert, and the Hubbert peak theory of Peak oil production. He also gave the Hadean geologic eon, Earth's earliest, its name, using the Greek word for the Underworld to refer to a molten state of constant heat.

Cloud was the author of over 200 scientific and lay publications. Notably, his 1978 book, Cosmos, Earth, and Man: A Short History of the Universe, written for a general readership, has been called 'one of the first and finest presentations of "a more ample and more coherent picture of the world"' (Alles), and his 1988 Oasis in Space: Earth History from the Beginning a "comprehensive work of synthesis and reflection... a documented history of the earth and life on it... and an impressive capstone to his remarkable scholarship" (Crowell).

One of Cloud's scientific heirs, his nephew John P. Grotzinger, was the Project Scientist for the NASA Mars Science Laboratory Curiosity rover Mission Team.

In 1990 special volume titled “Proterozoic Evolution and Environments” was published in the American Journal of Science in honour of Cloud's works.