Will Keith Kellogg (born William Keith Kellogg; April 7, 1860 – October 6, 1951) was an American industrialist in food manufacturing, who founded the Kellogg Company, which produces a wide variety of popular breakfast cereals. He was a member of the Seventh-day Adventist Church and practiced vegetarianism as a dietary principle taught by his church. He also founded the Kellogg Arabian Ranch, which breeds Arabian horses. Kellogg was a philanthropist and started the Kellogg Foundation in 1934 with a $66-million donation.

As a young businessman, Kellogg started out selling brooms in his hometown of Battle Creek, Michigan. In December 1878, W.K. Kellogg was hired by George H. King at the urging of James Springer White, also known as Elder White, to help run his new broom factory in Dallas, Texas. W.K. returned home in November 1879 to help his brother John Harvey Kellogg manage the Battle Creek Sanitarium. The sanitarium, originally the Western Health Reform Institute, was part of a pioneering effort based on the health principles advocated by the Seventh-day Adventist Church. The Kellogg family are of paternal English descent which can be traced to three brothers Daniel, Joseph and Samuel, of Braintree, Essex, England, who emigrated to the Connecticut colony in the 17th century.

John Kellogg described the Sanitarium system as "a composite physiologic method comprising hydrotherapy, phototherapy, thermotherapy, electrotherapy, mechanotherapy, dietetics, physical culture, cold-air cure, and health training".

The Kelloggs pioneered the process of making flaked cereal. Because of the commercial potential of the discovery, W.K. wanted it kept a secret. However, John allowed anyone in the sanitarium to observe the flaking process and one sanitarium guest, C. W. Post, copied the process to start his own company. That company became Post Cereals and, later, General Foods, the source of Post's first million dollars. That upset W.K. to the extent that he left the sanitarium to create his own company.

Together with his brother John, W.K. Kellogg promoted cereals, especially corn flakes (maize), as a healthy breakfast food. They started the Sanitas Food Company around 1897, focusing on the production of their whole-grain cereals. At the time, the standard breakfast for the well-off was eggs and meat, while the poor ate porridge, farina, gruel and other boiled grains. The brothers eventually argued over the addition of sugar to their product, with W.K. in favour of adding sugar and John opposed. In 1906, Will founded the Battle Creek Toasted Corn Flake Company, which later became the Kellogg Company.

In 1930, he established the W. K. Kellogg Foundation, ultimately donating $66 million to it. His company was one of the first to put nutrition labels on foods. He also offered the first inside-the-box prize for children. Kellogg said, "I will invest my money in people."

During the Great Depression, Kellogg directed his cereal plant to work four shifts, each lasting six hours. This gave more people in Battle Creek the opportunity to work during that time.

Kellogg had a longtime interest in Arabian horses. In 1925, he purchased 377 acres (153 hectares) for $250,000 in Pomona, California, to establish an Arabian horse ranch. Starting with breeding stock descended from the imports of Homer Davenport and W. R. Brown, Kellogg then looked to England, where he purchased a significant number of horses from the Crabbet Arabian Stud, making multiple importations during the 1920s. The Kellogg ranch became well known in southern California not only for its horse breeding program but also for its entertaining, weekly horse exhibitions, open to the public and frequently visited by assorted Hollywood celebrities. Among many other connections to Hollywood, the actor Rudolph Valentino borrowed the Kellogg stallion "Jadaan" for his 1926 movie Son of the Sheik, along with a Kellogg employee, Carl Raswan, who rode in certain scenes as Valentino's stunt double.