The United Service Organizations Inc. (USO) is an American nonprofit-charitable corporation that provides live entertainment, such as comedians, actors and musicians, social facilities, and other programs to members of the United States Armed Forces and their families. Since 1941, it has worked in partnership with the Department of War, and later with the Department of Defense (DoD), relying heavily on private contributions and on funds, goods, and services from various corporate and individual donors. Although it is congressionally chartered, it is not a government agency.

Founded during World War II, the USO sought to be the GI's "home away from home" and began a tradition of entertaining the troops and providing social facilities. Involvement in the USO was one of the many ways in which the nation had come together to support the war effort, with nearly 1.5 million people having volunteered their services in some way. The USO initially disbanded in 1947, but was revived in 1950 for the Korean War, after which it continued, also providing peacetime services. During the Vietnam War, USO social facilities ("USOs") were sometimes located in combat zones.

The organization became particularly known for its live performances, called "camp shows", through which the entertainment industry helps boost the morale of servicemen and women. In the early days, Hollywood was eager to show its patriotism, and many celebrities joined the ranks of USO entertainers. They went as volunteers to entertain, and celebrities continue to provide volunteer entertainment in military bases in the U.S. and overseas, sometimes placing their own lives in danger by traveling or performing under hazardous conditions. In 2011, the USO was awarded the National Medal of Arts.

According to the organization's website, the USO maintains over 250 locations worldwide. During a gala marking the USO's 75th anniversary in 2016, retired Army Gen. George W. Casey Jr., then-chairman of the USO Board of Governors, estimated that the USO has served more than 35 million Americans over its history.

The USO built on and greatly enriched a tradition of entertaining troops in the war theater. In World War I, many soldiers were treated to a variety of theatrical and musical performances (in which some soldiers participated when professionals were scarce). And in the Civil War, soldiers themselves typically played instruments and sang songs for entertainment. There were a few traveling entertainers who risked their lives to entertain troops in camp. The most well known, indeed famous, of these was Father Locke, whose service was requested by President Abraham Lincoln himself.

The USO was founded on February 4, 1941 by Mary Ingraham in response to a request from President Franklin D. Roosevelt to provide morale and recreation services to U.S. uniformed military personnel. Roosevelt was elected as its honorary chairman. This request brought together six civilian organizations: the Salvation Army, YMCA, Young Women's Christian Association (YWCA), National Catholic Community Service, National Travelers Aid Association and the National Jewish Welfare Board. They were brought together under one umbrella to support U.S. troops, as opposed to operating independently as some had done during the First World War. Roosevelt said he wanted "these private organizations to handle the on-leave recreation of the men in the armed forces." According to historian Emily Yellin, "The government was to build the buildings and the USO was to raise private funds to carry out its main mission: boosting the morale of the military."

The first national campaign chairman was Thomas Dewey, who raised $16 million in the first year. The second chairman was future senator Prescott Bush. The USO was incorporated in New York on February 4, with the first facility erected in DeRidder, Louisiana, 1941. More USO centers and clubs opened around the world as a "Home Away from Home" for GIs. The USO club was a place to go for dances and social events, for movies and music, for a quiet place to talk or write a letter home, or for a free cup of coffee and an egg.

The USO also brought Hollywood celebrities and volunteer entertainers to perform for the troops. According to movie historian Steven Cohan, "most of all ... in taking home on the road, it equated the nation with showbiz. USO camp shows were designed in their export to remind soldiers of home." They did this, he noted, by "nurturing in troops a sense of patriotic identification with America through popular entertainment." An article in Look magazine at the time, stated, "For the little time the show lasts, the men are taken straight to the familiar Main Street that is the goal of every fighting American far away from home." Maxene Andrews wrote, "The entertainment brought home to the boys. Their home." Actor George Raft stated at the beginning of the war, "Now it's going to be up to us to send to the men here and abroad real, living entertainment, the songs, the dances, and the laughs they had back home."