The United Steel Workers of America (USWA) claims over 1.2 million active and retired workers amongst its ranks, including many workers outside of the steel industry despite the union's name. It is an active labor union, working politically and in workplaces to promote its ideals and advocate for its workers.

The USWA was established May 22, 1942 by a convention of representatives from the old Amalgamated Association of Iron, Steel, and Tin Workers and the Steel Workers Organizing Commitee, after almost six years of violent and divisive struggles to create a new union of steelworkers. The drive to create the USWA included such violent incidents as the infamous Memorial Day, 1937, when Chicago policemen fired on workers outside of Republic Steel and killed ten men.

The 46,000 members of the Aluminum Workers of America voted to merge with the budding steelworker union that was the USWA on June 30, 1944. Eventually, eight more unions joined the USWA as well: the International Union of Mine, Mill and Smelter Workers (1967); the United Stone and Allied Product Workers of America (1971); District 50, the Allied and Technical Workers of America (1972); the Upholsterers International Union of North America (1985); the United Rubber, Cork, Linoleum & Plastic Workers of America (URW) (1995); the Aluminum, Brick and Glass Workers Union (ABG) (1996); the Canadian Division of the Transportation Communications International Union (1999); and the American Flint Glass Workers Union (AFGWU) (2003).

On January 11, 2005, the union announced a merger with the Paper, Allied-Industrial, Chemical and Energy Workers International Union (PACE). The new union, with 850,000 active members in the United States and Canada, will be the largest industrial labor union in North America. The union will be known as the United Steel, Paper and Forestry, Rubber, Manufacturing, Energy, Allied-Industrial and Service Workers International Union, or by the acronym USW.

Founder and first president of the USWA Philip Murray led the union through its inception drive and dangerous first decade, when the workers of USWA went on strike several times to win concessions such as the right to collective bargaining with steel companies and paid vacations.

The current international president of the United Steel Workers of America is Leo W. Gerard, who has served as president since 2001.