Collier's Weekly was a United States magazine that was published between 1888 and 1957.

The periodical was founded as Collier's Once a Week in April 1888 as a magazine of "fiction, fact, sensation, wit, humor, news" by Peter Collier (1849-1918), an Irish immigrant who left Ireland when he was seventeen years old and founded a company producing books for the Roman Catholic market. By 1892 it had a circulation of over 250,000 and was one of largest selling magazines in the United States.

In 1895 its name was changed to Collier's Weekly: An Illustrated Journal. The magazine decided to focus primarily on news and became a leading exponent of the half-tone news picture. To fully exploit this new technology, Peter Collier recruited James H. Hare, one of the pioneers of photojournalism.

Norman Hapgood became editor of Collier's Weekly in 1903 and attracted many leading writers. In May, 1906, he commissioned Jack London to report on the San Francisco earthquake. His report was accompanied by sixteen pages of pictures.

Under Hapgood's guidance, Collier's Weekly began publishing the work of investigative journalists such as Ida Tarbell, C.P. Connolly, Samuel Hopkins Adams and Ray Stannard Baker. It had great impact, resulting in such changes as the reform of the child labor laws, slum clearance and women's suffrage. In April 1905, an article by Upton Sinclair, "Is Chicago Meat Clean?", helped to persuade the Senate to pass the 1906 Meat Inspection Act.

In October 1905, Adams contributed a series of eleven articles, "The Great American Fraud". Adams analyzed the contents of some of the country's most popular patent medicines, and pointed out that the companies producing these medicines were making false claims about their products, and that in some cases, these medicines were actually damaging the health of those people using them. The series had a tremendous impact on public opinion and resulted in the passing of the first Pure Food and Drug Act (1906).

As a result of this pioneering investigative journalism, Collier's Weekly established reputation as a proponent of social reform. When attempts by various companies to sue Peter Collier ended in failure, other magazines became involved in what Theodore Roosevelt described unflatteringly as "muckraking journalism."

Norman Hapgood left for Harper's Weekly in 1912. Robert Collier, son of the founder, became the new editor. Circulation continued to grow and by 1917 circulation had reached a million every week.

By the late 1920s Collier's Weekly began to concentrate on the serialization of novels. Produced in about ten parts, the magazine ran two novels at a time. Non-fiction was also serialized, including an account of the First World War by Winston Churchill. In the 1930s, Churchill was a regular contributor to Collier's but this came to an end in 1938 when he became a minister in the British government.

During the Second World War the circulation of Collier's had reached two and a half million, partly due to its publishing writers such as Martha Gellhorn and Ernest Hemingway, who reported on the Spanish Civil War.

Collier's was also known for its groundbreaking series of articles about space flight published in the early 1950s called "Man Will Conquer Space Soon".

Circulation of Collier's began to fall after the war and in August 1953 it changed from a weekly to a biweekly magazine. Collier's continued to lose money and in January 1957 it closed down.