Harlingen ( ) is a city in Cameron County in the heart of the Rio Grande Valley of south Texas, United States, about from the coast of the Gulf of Mexico. The city covers more than and is the second largest city in Cameron County, as well as the sixth largest in the Rio Grande Valley. As of the 2010 census the city had a population of 64,849, for a growth rate of 12.5% since the 2000 census. It is the city with the least expensive cost of living in the United States.