Collin County is a county in the northern part of the U.S. state of Texas. It is part of the Dallas–Fort Worth–Arlington metropolitan statistical area, and a small portion of the city of Dallas is in the county. At the 2020 United States census, the county's population was 1,064,465, making it Texas's sixth-most populous county and the 43rd-largest county by population in the United States. Its county seat is McKinney.