San Justo is a city and the seat of government of La Matanza Partido, Buenos Aires Province, Argentina. It is a relatively prosperous city enjoying a variety of commercial and cultural resources. Its strategic location on National Highway 3 and Provincial Highway 4 (the Greater Buenos Aires beltway), make it among the most active, for its size, in the county.

Pedro Bonifacio Palacios, an Argentine writer, was born in San Justo, and one of the main streets of the city is named in his honor under his nickname Almafuerte. The Plaza San Martin is the center of La Matanza and its civic center, where the main institutions of the partido are located within a four-block radius.