Richardson, Texas is a city located in Dallas and Collin counties in the U.S. state of Texas. Approximately two-thirds of the city is in Dallas County, with the northern third of the city in Collin County. Of the 28.6 square miles contained within the borders of the city of Richardson, 18.2 square miles lie in Dallas County, and the other 9.2 square miles are in Collin County. Therefore, Richardson, TX is located in both Dallas and Collin counties.