Piedras Negras is a city and seat of the surrounding municipality of the same name in the Mexican state of Coahuila. It stands at the northeastern edge of Coahuila on the U.S.-Mexico border, across the Río Bravo (Rio Grande) from Eagle Pass in the U.S. state of Texas. In the 2010 census the city had a population of 150,178 people, which accounted for 98.28 percent of its municipality's total population of 152,806. The municipality's area is relatively small, but does include some outlying minor localities not located inside the city limits. The Eagle Pass and Piedras Negras area is connected by the Eagle Pass-Piedras Negras International Bridge, Camino Real International Bridge, and the Eagle Pass Union Pacific International Railroad Bridge.
The city was founded in 1849. It was renamed Ciudad Porfirio Díaz in 1888, in homage to President Porfirio Díaz, but reverted to its original name following the Mexican Revolution.
In Spanish Piedras Negras translates to "black stones" – a reference to the coal deposits that exist in the area. Across the river, coal was formerly mined on the US side at Dolchburg, near Eagle Pass. This mine closed around 1905, after a fire. Mexico currently operates two large coal-fired power plants named "José López Portillo" and "Carbón 2" located 30 miles (48 km) south of Piedras