Warner Bros. Entertainment Inc. (WBEI), commonly known as Warner Bros. (WB), is an American filmed entertainment and media corporation headquartered at the Warner Bros. Studios complex in Burbank, California, and the main namesake subsidiary of Warner Bros. Discovery. Founded on April 4, 1923, by four brothers, Harry, Albert, Sam and Jack Warner, the company established itself as a leader in the American film industry before diversifying into animation, television, and video games. It is one of the "Big Five" major American film studios and a member of the Motion Picture Association (MPA).

The company is known for its film studio division, the Warner Bros. Motion Picture Group, which includes Warner Bros. Pictures, New Line Cinema, Warner Bros. Pictures Animation, Castle Rock Entertainment and the Warner Bros. Television Group. Bugs Bunny, a character created for the Looney Tunes series, is the company's official mascot.

The company's name originated from the founding Warner brothers (born Wonsal, Woron, and Wonskolaser before Anglicization): Harry, Albert, Sam, and Jack Warner. Harry, Albert and Sam emigrated as young children with their Polish-Jewish mother to the United States from Krasnosielc, Poland (then part of Congress Poland within the Russian Empire), in October 1889, a year after their father emigrated to the U.S. and settled in Baltimore, Maryland. As in many other immigrant families, the elder Wonsal children gradually acquired anglicized versions of their Yiddish-sounding names: Szmuel Wonsal became Samuel Warner (nicknamed "Sam"), Hirsz Wonsal became Harry Warner, and Aaron Wonsal (although born with a given name common in the Americas) became Albert Warner. Jack, the youngest brother, was born in London, Ontario, during the family's two-year residency in Canada.

The three elder brothers began in the movie theater business, having acquired a movie projector with which they showed films in the mining towns of Pennsylvania and Ohio. In the beginning, Sam and Albert Warner invested $150 to present Life of an American Fireman and The Great Train Robbery. They opened their first theater, the Cascade, in New Castle, Pennsylvania, in 1903. When the original building was in danger of being demolished, the modern Warner Bros. called the current building owners and arranged to save it. The owners noted people across the country had asked them to protect it for its historical significance.

In 1904, the Warners founded the Pittsburgh-based Duquesne Amusement & Supply Company, to distribute films. In 1912, Harry Warner hired an auditor named Paul Ashley Chase. By the time of World War I, they had begun producing films; in the early 1920s they acquired their first studio facilities on Sunset Boulevard in Hollywood. Sam and Jack produced the pictures, while Harry and Albert, along with their auditor and now-controller Chase, handled finance and distribution in New York City. During World War I their first nationally syndicated film, My Four Years in Germany, based on a popular book by former ambassador James W. Gerard, was released. On April 4, 1923, with help from money loaned to Harry by his banker Motley Flint, they formally incorporated as Warner Bros. Pictures, Incorporated. (As late as the 1960s, Warner Bros. claimed 1905 as its founding date.)

The first important deal was the acquisition of the rights to Avery Hopwood's 1919 Broadway play, The Gold Diggers, from theatrical impresario David Belasco. However, Rin Tin Tin, a dog brought from France after World War I by an American soldier, established their reputation. Rin Tin Tin's third film was the feature Where the North Begins, which was so successful that Jack signed the dog to star in more films for $1,000 per week. Rin Tin Tin became the studio's top star. Jack nicknamed him "The Mortgage Lifter" and the success boosted Darryl F. Zanuck's career. Zanuck eventually became a top producer and between 1928 and 1933 served as Jack's right-hand man and executive producer, with responsibilities including day-to-day film production. More success came after Ernst Lubitsch was hired as head director; Harry Rapf left the studio to join Metro-Goldwyn-Mayer. Lubitsch's film The Marriage Circle was the studio's most successful film of 1924, and was on The New York Times best list for that year.

Despite the success of Rin Tin Tin and Lubitsch, Warner's remained a lesser studio. Sam and Jack decided to offer Broadway actor John Barrymore the lead role in Beau Brummel. The film was so successful that Harry signed Barrymore to a long-term contract; like The Marriage Circle, Beau Brummel was named one of the ten best films of the year by the Times. By the end of 1924, Warner Bros. was arguably Hollywood's most successful independent studio, where it competed with "The Big Three" Studios (First National, Paramount Pictures, and Metro-Goldwyn-Mayer (MGM)). As a result, Harry Warner—while speaking at a convention of 1,500 independent exhibitors in Milwaukee, Wisconsin—was able to convince the filmmakers to spend $500,000 in newspaper advertising, and Harry saw this as an opportunity to establish theaters in places such as New York City and Los Angeles.

As the studio prospered, it gained backing from Wall Street, and in 1924 Goldman Sachs arranged a major loan. With this new money, the Warners bought the pioneer Vitagraph Company which had a nationwide distribution system. In 1925, Warners' also experimented in radio, establishing a successful radio station, KFWB, in Los Angeles.