Duncanville is a city in southwestern Dallas County, Texas, United States. Duncanville's population was 40,706 at the 2020 census. The city is part of the Best Southwest area, which includes Duncanville, Cedar Hill, DeSoto, and Lancaster.

Dallas County is the second-most populous county in the U.S. state of Texas with a 2020 U.S. census of 2,613,539, making it the ninth-most populous county in the country. Dallas County is included in the Dallas-Arlington-Fort Worth metropolitan statistical area—colloquially referred to as the Dallas–Fort Worth metroplex. Municipal expansion within Dallas County has blurred the geographic lines between cities and between neighboring counties.