Matamoros, officially known as Heroica Matamoros, is a city in the northeastern Mexican state of Tamaulipas, and the municipal seat of the homonymous municipality. It is on the southern bank of the Rio Grande, directly across the border from Brownsville, Texas, United States. Matamoros is the second largest city in the state of Tamaulipas. As of 2016, Matamoros had a population of 520,367.