The International Brotherhood of Teamsters (IBT) is a labor union in the United States and Canada. Formed in 1903 by the merger of the Team Drivers International Union and the Teamsters National Union, the union now represents a diverse membership of blue- and white-collar workers in both the public and private sectors, totalling about 1.3 million members in 2015. The union was formerly called the International Brotherhood of Teamsters, Chauffeurs, Warehousemen and Helpers of America.

The American Federation of Labor (AFL) had helped form local unions of teamsters since 1887. In November 1898, the AFL organized the Team Drivers' International Union (TDIU). In 1901, a group of teamsters in Chicago, Illinois, broke from the TDIU and formed the Teamsters National Union. Unlike the TDIU, which permitted large employers to be members, the new Teamsters National Union permitted only employees, teamster helpers, and owner-operators owning only a single team to join, and advocated higher wages and shorter hours more aggressively than the TDIU. Claiming more than 28,000 members in 47 locals, its president, Albert Young, applied for membership in the AFL. The AFL asked the TDIU to merge with Young's union to form a new, AFL-affiliated union and the two groups did so in 1903, forming the International Brotherhood of Teamsters (IBT), and electing Cornelius Shea as the new union's first president. The election process proved tumultuous. Shea effectively controlled the convention because the Chicago locals—representing nearly half the IBT's membership—supported his candidacy en bloc. Shea was opposed by John Sheridan, president of the Ice Drivers' Union of Chicago. Sheridan and George Innes, president of the TDIU, accused Shea of embezzlement in an attempt to prevent his election. Shea won the election on August 8, 1903, by a vote of 605 to 480. The new grouping elected Edward L. Turley of Chicago as secretary-treasurer and Albert Young as general organizer.

The union, like most unions within the AFL at the time, had a largely decentralized structure, with a number of local unions that governed themselves autonomously and tended to look only after their own interests in the geographical jurisdiction in which they operated. The teamsters were vitally important to the labor movement, for a strike or sympathy strike by the teamsters could paralyze the movement of goods throughout a city and bring a strike into nearly every neighborhood. It also meant that teamsters leaders were able to demand bribes in order to avoid strikes, and control of a teamsters local could bring organized crime significant revenues. During Shea's presidency, the entire teamsters union was notoriously corrupt.

Several major strikes occupied the union in its first three years. In November 1903, teamsters employed by the Chicago City Railway went out on strike. Shea attempted to stop sympathy strikes by other teamster locals, but three locals walked out and eventually disaffiliated over the sympathy-strike issue. A sympathy strike in support of 18,000 striking meat cutters in Chicago in July 1904 led to riots before the extensive use of strikebreakers led Shea to force his members back to work (leading to the collapse of the meat cutters' strike). In the midst of the strife in 1904, the teamsters convention in Cincinnati, Ohio, re-elected Shea by acclamation on August 8, 1904. Under his leadership, the union had expanded to nearly 50,000 members in 821 locals in 300 cities, making the Teamsters one of the largest unions in the United States.

In 1905, 10,000 teamsters struck in support of locked-out tailors at Montgomery Ward, and eventually more than 25,000 teamsters manned the picket lines. But when local newspapers discovered that Shea was living in a local brothel, kept a 19-year-old waitress as a mistress, and had spent the strike hosting parties, public support for the strike collapsed, and the strike ended on August 1, 1905. Despite the revelations, Shea won re-election on August 12, 1905, by a vote of 129 to 121.

Shea was re-elected again in 1905 and 1906, although significant challenges to his presidency occurred each time. Shea's first trial on charges stemming from the 1905 Montgomery Ward strike ended in a mistrial. However, during the 1906 re-election Shea had promised that he would resign the presidency once his trial had ended. But he did not, and most union members withdrew their support for him. Daniel J. Tobin of Boston was elected Shea's successor by a vote of 104 to 94 in August 1907.

Tobin was president of the Teamsters from 1907 to 1952. Although he faced opposition in his re-election races in 1908, 1909 and 1910, he never faced opposition again until his retirement in 1952.

The Teamsters began to expand dramatically and mature organizationally under Tobin. He pushed for the development of "joint councils" to which all local unions were forced to affiliate. Varying in geographical and industrial jurisdiction, the joint councils became important incubators for up-and-coming leadership and negotiating master agreements which covered all employers in a given industry. Tobin also actively discouraged strikes in order to bring discipline to the union and encourage employers to sign contracts, and founded and edited the union magazine, the International Teamster. Under Tobin, the Teamsters also first developed the "regional conference" system (developed by Dave Beck in Seattle), which provided stability, organizing strength, and leadership to the international union.