In 1899, the United States Department of War conducted a census of Puerto Rico finding that the population of Rosario barrio was 852.

Rosario is located in the Cerro Las Mesas, and borders the barrios of Juan Alonso and Limón in Mayagüez, and barrio Montoso of Las Marías, Puerto Rico on the north side.

To the east, it is also adjacent to the barrio Montoso of Las Marías and Oconuco Alto in San Germán.

Rosario covers an area of approximately 7,672 acres of land, which is equivalent to 11.64 square miles.

It is mainly a mountainous region, with rugged topography accounting for 75% of its surface.