Service Employees International Union (SEIU) is a labor union representing 2 million workers in over 100 occupations in the United States, Canada, and Puerto Rico. SEIU is focused on organizing workers in three sectors: healthcare (over half of members work in the healthcare field), including hospital, home care and nursing home workers; public services (government employees, including law enforcement); and property services (including janitors, security guards and food service workers).

SEIU has over 150 local branches. It is affiliated with the AFL-CIO, the Strategic Organizing Center, and the Canadian Labour Congress. SEIU's international headquarters is located in Washington, D.C., and it is one of the largest unions in the country.

The union is known for its strong support for Democratic candidates. It spent $28 million supporting Barack Obama in the 2008 presidential election. In 2012, SEIU was the top outside spender on Democratic campaigns, reporting almost $70 million of campaign donations, television ads and get-out-the-vote efforts in support of President Obama and other Democrats. SEIU is a major supporter of the Affordable Care Act and of increased minimum wage laws, including wage increases for fast food workers. The union is the primary backer of the Fight for $15.

The SEIU was founded on April 23, 1921 in Chicago as the Building Service Employees International Union (BSEIU); its first members were janitors, elevator operators, and window washers. The union's membership increased significantly with a 1934 strike in New York City's Garment District. In order to reflect its increasingly diversified membership, in 1968 the union renamed itself Service Employees International Union. In 1980 through 1984, most of the SEIU's growth came from mergers with four other unions, including the International Jewelry Workers' Union and the Drug, Hospital, and Health Care Employees Union.

In 1995, SEIU President John Sweeney was elected president of the AFL–CIO, the main confederation of labor unions in the United States. After Sweeney's departure, former social worker Andrew Stern was elected president of SEIU. In the first ten years of Stern's administration, the union's membership grew rapidly and the SEIU became the largest union in the AFL–CIO.

In 2003, SEIU was a founding member of the New Unity Partnership, an organization of unions that pushed for a greater commitment to organizing unorganized workers into unions. In 2005, SEIU was a founding member of the Change to Win Coalition, which furthered the reformist agenda. Their differences with AFL-CIO boiled over on the eve of the 2005 AFL–CIO convention, as the SEIU and Teamsters announced that they were disaffiliating from the AFL–CIO, and boycotting the convention. The Change to Win Federation held its founding convention in September 2005, where SEIU Secretary-Treasurer Anna Burger was announced as the organization's chair.

In the following decade, several Change to Win members disaffiliated and re-joined the AFL–CIO, leaving SEIU, the Teamsters, and the United Farm Workers as the remaining members. The SEIU's decision to break away from the AFL–CIO is considered controversial by some labor experts. After the disaffiliation, the SEIU continued to experience significant growth in membership. Stern stepped down as president of SEIU in 2010, and was replaced by Mary Kay Henry, a long-time organizer and staff member at the union, and its first female president.

April Verrett was elected the union's first Black president in May 2024.