cabecera (county seat) of La Matanza partido (county), Gran (Greater) Buenos Aires, eastern Argentina. It lies directly southwest of the city of Buenos Aires, in Buenos Aires provincia (province). In 1856 the poblador (person granted land as an incentive to settle an area) of San Justo, José Gorsch, donated the land for the founding of the town on behalf of the Justo Villegas family. On December 25 of the same year, the town of San Justo was founded. In 1860 the first church was established in the town.

The town of San Justo was declared a city in 1955. By this time it had become an important industrial centre, manufacturing plows, automobile accessories, paper, and rubber goods. On agricultural lands farther west and south, wheat, corn (maize), barley, and alfalfa are grown and livestock is raised.

San Justo, and La Matanza county in particular, experienced rapid growth in the 1960s and 1970s. Pop. (2001) 105,274.