John Harvey Kellogg (February 26, 1852 – December 14, 1943) was an American businessman, inventor, and physician who was an advocate of theological modernism and 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 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." Popular misconceptions falsely attribute various cultural practices, inventions, and historical events to Kellogg.

Kellogg was an advocate of theological modernism and held beliefs different from that of traditional Nicene Christianity. He rejected original sin, inherent human depravity, and held non-Christocentric views, viewing Jesus' atonement as "his exemplary life" on Earth rather than on the Cross. Becoming a Seventh-day Adventist as their beliefs shifted towards Trinitarianism during the 1890s, Adventists were "unable to accommodate the essentially liberal understanding of Christianity" exhibited by Kellogg, viewing his theology as pantheistic and unorthodox. Disagreements with other members of the SDA led to a major schism within the SDA : he was disfellowshipped in 1907, but continued to follow many of their beliefs and directed the sanitarium until his death. Kellogg helped to establish the American Medical Missionary College in 1895. The college operated independently until 1910, when it merged with Illinois State University.

As 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 enemas to clear "intestinal flora", exercise, sun-bathing, and hydrotherapy, as well as the abstention from smoking tobacco, drinking alcoholic beverages, and sexual activity. Kellogg dedicated the last 30 years of his life to promoting eugenics. He co-founded the Race Betterment Foundation, co-organized several National Conferences on Race Betterment and attempted to create a 'eugenics registry'. Alongside discouraging 'racial mixing', Kellogg was in favor of sterilizing 'mentally defective persons', promoting a eugenics agenda while working on the Michigan Board of Health and helping to enact authorization to sterilize those deemed 'mentally defective' into state laws during his tenure. 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, promoting 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. This creation of the modern breakfast cereal changed "the American breakfast landscape forever."