Los Fresnos is a city in Cameron County, Texas, United States. The population was 5,542 at the 2010 census. It is part of the Brownsville–Harlingen–Raymondville and the Matamoros–Brownsville metropolitan areas.

Located in south central Cameron County, the city was named for the fresnos (Fraxinus berlandieriana) scattered in the woods and along streams. In 1912, Lon Hill, an early land owner, formed a company to develop a canal system which would use the Rio Grande to irrigate land for farming. Railroad construction in the early 1900s began to bring settlers to the area, and by 1915 a post office was established. Because of the fertile, irrigated land, farm products increased and with the long growing season in the Rio Grande Valley, farming flourished. Today the city is still surrounded by fertile farm/ranch land. Major crops are cotton, sugarcane, grains, orange, and red grapefruit orchards.

The U.S. Immigration and Customs Enforcement (ICE) Port Isabel Detention Center [1] is located in Los Fresnos.

The Los Fresnos PRCA Rodeo & Livestock Show held every year the first week in February.

The Los Fresnos Motocross Park has camp sites available at the track to accommodate RV's & Tents. Quads are also welcomed to race. There are categories for every age and bike sizes, such as Mini 50cc, 250cc, Quads, etc.