John Harvey Kellogg (February 26, 1852 – December 14, 1943) was an American businessman, inventor, physician, and advocate of the Progressive Movement. He was the director of the Battle Creek Sanitarium in Battle Creek, Michigan, founded by members of the Seventh-day Adventist Church. It combined aspects of a European spa, a hydrotherapy institution, a hospital, and a high-class hotel. Kellogg treated the rich and famous, as well as the poor who could not afford other hospitals. According to Encyclopædia Britannica, his "development of dry breakfast cereals was largely responsible for the creation of the flaked-cereal industry, with the founding and the culmination of the global conglomeration brand of Kellogg's (now Kellanova)."

An early proponent of the germ theory of disease, Kellogg was well ahead of his time in relating intestinal flora and the presence of bacteria in the intestines to health and disease. The sanitarium approached treatment in a holistic manner, actively promoting vegetarianism, nutrition, the use of yogurt enemas to clear "intestinal flora", exercise, sun-bathing, and hydrotherapy, as well as abstinence from smoking tobacco, drinking alcoholic beverages, and sexual activity. Kellogg dedicated the last 30 years of his life to promoting eugenics and racial segregation. Kellogg was a major leader in progressive health reform, particularly in the second phase of the clean living movement. He wrote extensively on science and health. His approach to "biologic living" combined scientific knowledge with Adventist beliefs and the promotion of health reform and temperance. Many of the vegetarian foods that Kellogg developed and offered his patients were publicly marketed: Kellogg's brother, Will Keith Kellogg, is best known today for the invention of the breakfast cereal corn flakes.

Kellogg held liberal Christian theological beliefs radically different from mainstream Nicene Christianity and emphasized what he saw as the importance of human reason over many aspects of traditional doctrinal authority. He strongly rejected fundamentalist and conservative notions of original sin, human depravity, and the atonement of Jesus, viewing the last in terms of "his exemplary life" on Earth rather than death. Kellogg became a Seventh-day Adventist (SDA) as the group's beliefs shifted towards Trinitarianism during the 1890s, and Adventists were "unable to accommodate the essentially liberal understanding of Christianity" exhibited by Kellogg, viewing his theology as pantheistic and unorthodox. His disagreements with other members of the SDA Church led to a major schism: he was disfellowshipped in 1907, but continued to adhere to many of the church's beliefs and directed the sanitarium until his death. Kellogg helped to establish the American Medical Missionary College in 1895. Popular misconceptions have wrongly attributed various cultural practices, inventions, and historical events to Kellogg.