Dallas County is a county located in the central part of the U.S. state of Alabama. As of the 2020 census, its population was 38,462. The county seat is Selma. Its name is in honor of United States Secretary of the Treasury Alexander J. Dallas, who served from 1814 to 1816.

Dallas County comprises the Selma, AL Micropolitan Statistical Area.

Dallas County was created by the Alabama territorial legislature on February 9, 1818, from Montgomery County. This was a portion of the Creek cession of lands to the US government of August 9, 1814. The Creek were known as one of the Five Civilized Tribes of the Southeast. The county was named for U.S. Treasury Secretary Alexander J. Dallas of Pennsylvania.

Dallas County is located in what has become known as the Black Belt region of the west-central portion of the state. The name referred to its fertile soil, and the area was largely developed for cotton plantations, worked by numerous enslaved African Americans in the antebellum period. After emancipation following the Civil War, many of the African Americans stayed in the area and worked as sharecroppers and tenant farmers. The county has been majority black since before the war.

Dallas County produced more cotton by 1860 than any other county in the state, requiring a large supply of workers, who were mostly enslaved. Dallas County slave owners on average had seventeen enslaved workers (compared to ten in Montgomery County, for instance); slave owners made up some 16% of the county's white population, but if their families are added, at least a third of the county's population was attached to a slaveholding family, according to historian Alston Fitts.

Well-known local slaveowners include Washington Smith, owner of a big plantation in Bogue Chitto, Alabama, near Selma, and founder of the Bank of Selma. After Emancipation he continued to exert great influence over the African-American people in the county. Shortly before the war, Smith had bought a West African girl, Redoshi, one of an illegal shipment of slaves in 1860. He called her Sally Smith. She was from Benin, kidnapped at age 12 and one of numerous African captives transported on the Clotilda to Mobile, Alabama, more than 50 years after the slave trade had been abolished.

The county is traversed by the Alabama River, flowing from northeast to southwest across the county. It is bordered by Perry, Chilton, Autauga, Lowndes, Wilcox, and Marengo counties. Originally, the Dallas county seat was at Cahaba, which also served as the state capital for a brief period. In 1865, the county seat was transferred to Selma, Alabama as the center of population had moved. Other towns and communities in the still mostly rural county include Marion Junction, Sardis, Orrville, Valley Grande, and Minter.

Cotton production suffered in the early 20th century due to infestation of boll weevil, which invaded cotton areas throughout the South. At the turn of the 20th century, the state legislature disenfranchised most blacks and many poor whites through provisions of a new state constitution requiring payment of poll tax and passing a literacy test for voter registration. These largely survived legal challenges and blacks were excluded from the political system.