The color line, also known as the color barrier, in American baseball excluded players of black African descent from Major League Baseball and its affiliated Minor Leagues until 1947 (with a few notable exceptions in the 19th century before the line was firmly established). Racial segregation in professional baseball was sometimes called a gentlemen's agreement, meaning a tacit understanding, as there was no written policy at the highest level of organized baseball, the major leagues. A high minor league's vote in 1887 against allowing new contracts with black players within its league sent a powerful signal that eventually led to the disappearance of blacks from the sport's other minor leagues later that century, including the low minors.

After the line was in virtually full effect in the early 20th century, many black baseball clubs were established, especially during the 1920s to 1940s when there were several Negro leagues. During this period Native Americans, and native Hawaiians (e.g. Prince Oana) were able to play in the Major Leagues.The color line was broken for good when Jackie Robinson signed with the Brooklyn Dodgers organization for the 1946 season. In 1947, both Robinson in the National League and Larry Doby with the American League's Cleveland Indians appeared in games for their teams. By the late 1950s, the percentage of black players on Major League teams matched or exceeded that of the general population.

Jack Roosevelt Robinson (January 31, 1919 – October 24, 1972) was an American professional baseball player who became the first African American to play in Major League Baseball (MLB) in the modern era. Robinson broke the baseball color line when he started at first base for the Brooklyn Dodgers on April 15, 1947. When the Dodgers signed Robinson, it heralded the end of racial segregation in professional baseball that had relegated black players to the Negro leagues since the 1880s. Robinson was inducted into the Baseball Hall of Fame in 1962.During his 10-year MLB career, Robinson won the inaugural Rookie of the Year Award in 1947, was an All-Star for six consecutive seasons from 1949 through 1954, and won the National League Most Valuable Player Award in 1949—the first black player so honored. Robinson played in six World Series and contributed to the Dodgers' 1955 World Series championship.

In 1997, MLB retired his uniform number 42 across all major league teams; he was the first professional athlete in any sport to be so honored. MLB also adopted a new annual tradition, "Jackie Robinson Day", for the first time on April 15, 2004, on which every player on every team wears No. 42.

Robinson's character, his use of nonviolence, and his talent challenged the traditional basis of segregation that had then marked many other aspects of American life. He influenced the culture of and contributed significantly to the civil rights movement. Robinson also was the first black television analyst in MLB and the first black vice president of a major American corporation, Chock full o'Nuts. In the 1960s, he helped establish the Freedom National Bank, an African-American-owned financial institution based in Harlem, New York. After his death in 1972, Robinson was posthumously awarded the Congressional Gold Medal and Presidential Medal of Freedom in recognition of his achievements on and off the field.