Macmillan Inc. (also known as Macmillan US, and formerly The Macmillan Company) was an American book publishing company originally established as the American division of the British Macmillan Publishers. The two were later separated and acquired by other companies, with the remnants of the original American division of Macmillan present in McGraw-Hill Education's Macmillan/McGraw-Hill textbooks, Gale's Macmillan Reference USA division, and some trade imprints of Simon & Schuster (Scribner, Free Press, and Atheneum Books) that were transferred when both companies were owned by Paramount Communications.

The German publisher Holtzbrinck, which bought the British Macmillan in 1999, purchased American rights to the Macmillan name in 2001 and rebranded its American division with it in 2007.

George Edward Brett opened the first Macmillan office in the United States in 1869. Macmillan sold its U.S. operations to the Brett family, George Platt Brett Sr. and George Platt Brett Jr. in 1896, resulting in the creation of an American company, Macmillan US (in which Macmillan Publishers held stake until 1951). Even with the split of the American company from its parent company in Britain, George Brett Jr. and Harold Macmillan remained close personal friends.

George P. Brett Jr. made the following comments in a letter dated 23 January 1947 to Daniel Macmillan about his family's devotion to the American publishing industry:

For the record my grandfather was employed by Macmillan's of England as a salesman. He came to the United States with his family in the service of Macmillan's of England and built up a business of approximately $50,000 before he died. He was succeeded . . . by my father, who eventually incorporated The Macmillan Company of New York and built up business of about $9,000,000. I succeeded my father, and we currently doing a business of approximately $12,000,000. So then, the name of Brett and the name of Macmillan have been and are synonymous in the United States.

Under the leadership of the Brett family, Macmillan served as the publisher of American authors Winston Churchill, Margaret Mitchell, author of Gone with the Wind, and Jack London, author of White Fang and Call of the Wild.

The Bretts remained in control of the American offices of Macmillan from its creation in 1869 to the early 1960s, "a span matched by few other families in the history of United States business."

Macmillan Publishers sold its stake in Macmillan US in 1951 and later re-entered the American publishing industry with the founding of St. Martin's Press in 1952.