The color line in American baseball excluded players of black African descent from Major League Baseball and affiliated minor leagues, until Jackie Robinson signed with the Brooklyn Dodgers organization for the 1946 season. Racial segregation in professional baseball is sometimes called a gentlemen's agreement, meaning a tacit understanding, because there was no written policy at the highest level of baseball organization. Some leagues did rule against member clubs signing black players, however, as the color line was drawn during the 1880s and 1890s.

On the "other side" of the color line, many black baseball clubs were established and especially during the 1920s to 1940s there were several "Negro" or "Colored" Leagues in operation, which primarily featured those players barred from organized baseball. Some light-skinned Hispanic players, some Native Americans, and native Hawaiians played white baseball during that period.

Formal beginning of segregation followed the baseball season of 1867. On October 16, the Pennsylvania State Convention of Baseball in Harrisburg denied the colored Pythian Baseball Club. Two months later the National Association of Base Ball Players decided to ban "any club including one or more colored persons." As baseball made the transition toward becoming a professional sport over the next decade, and the NABBP dissolved into competing organizations in 1871, professional players were no longer restricted by this rule and, for a short while – in 1878 and again in 1884 – African American players played professional baseball. Over time, they were slowly excluded more and more. As prominent players such as Cap Anson steadfastly refused to take the field with or against teams with African Americans on the roster, it became informally accepted that African Americans were not to participate in Major League Baseball.

Color line may refer to:

Jack Roosevelt "Jackie" Robinson (January 31, 1919 – October 24, 1972) was an American baseball player who became the first black Major League Baseball (MLB) player of the modern era. Robinson broke the baseball color line when he debuted with the Brooklyn Dodgers in 1947. As the first black man to play in the major leagues since the 1880s, he was instrumental in bringing an end to racial segregation in professional baseball, which had relegated black players to the Negro leagues for six decades. The example of his character and unquestionable talent challenged the traditional basis of segregation, which then marked many other aspects of American life, and contributed significantly to the Civil Rights Movement.

In addition to his cultural impact, Robinson had an exceptional baseball career. Over ten seasons, he played in six World Series and contributed to the Dodgers' 1955 World Championship. He was selected for six consecutive All-Star Games from 1949 to 1954, was the recipient of the inaugural MLB Rookie of the Year Award in 1947, and won the National League Most Valuable Player Award in 1949—the first black player so honored. Robinson was inducted into the Baseball Hall of Fame in 1962. In 1997, Major League Baseball retired his uniform number, 42, across all major league teams.