Jefferson County is the most populous county in the U.S. state of Alabama, located in the central portion of the state. As of the 2020 census, its population was 674,721. Its county seat is Birmingham. Its rapid growth as an industrial city in the 20th century, based on heavy manufacturing in steel and iron, established its dominance. Jefferson County is the central county of the Birmingham-Hoover, AL Metropolitan Statistical Area.

Jefferson County was established on December 13, 1819, by the Alabama Legislature. It was named in honor of former President Thomas Jefferson. The county is located in the north-central portion of the state, on the southernmost edge of the Appalachian Mountains. It is in the center of the (former) iron, coal, and limestone mining belt of the Southern United States.

Most of the original settlers were migrants of English ancestry from the Carolinas. Jefferson County has a land area of about 1,119 square miles (2,900 km²). Early county seats were established first at Carrollsville (1819 – 21), then Elyton (1821 – 73).

Founded around 1871, Birmingham was named for the industrial English city of the same name in Warwickshire. That city had long been a center of iron and steel production in Great Britain. Birmingham was formed by the merger of three towns, including Elyton. It has continued to grow by annexing neighboring towns and villages, including North Birmingham.

As Birmingham industrialized, its growth accelerated, particularly after 1890. It attracted numerous rural migrants, both black and white, for its new jobs. It also attracted European immigrants. Despite the city's rapid growth, for decades it was underrepresented in the legislature. Legislators from rural counties kept control of the legislature and, to avoid losing power, for decades refused to reapportion the seats or redistrict congressional districts. Birmingham could not get its urban needs addressed by the legislature.

Nearby Bessemer, Alabama, located 16 miles by car to the southwest, also grew based on industrialization. It also attracted many workers. By the early decades of the 20th century, it had a majority-black population, but whites dominated politically and economically.

Racial tensions increased in the cities and state in the late 19th century as whites worked to maintain white supremacy. The white-dominated legislature passed a new constitution in 1901 that disenfranchised most blacks and many poor whites, excluding them totally from the political system. While they were nominally still eligible in the mid-20th century for jury duty, they were overwhelmingly excluded by white administrators from juries into the 1950s. Economic competition among the new workers in the city also raised tensions. It was a rough environment of mill and mine workers in Birmingham and Bessemer, and the Ku Klux Klan was active in the 20th century, often with many police being members into the 1950s and 1960s.

In a study of lynchings in the South from 1877 to 1950, Jefferson County is documented as having the highest number of lynchings of any county in Alabama. White mobs committed 29 lynchings in the county, most around the turn of the century at a time of widespread political suppression of blacks in the state. Notable incidents include 1889's lynching of George Meadows.