Mercedes is a city in the province of Buenos Aires, Argentina. It is located 100 km west from Buenos Aires and 30 km south west of Luján. It is the head town of the district (partido) of Mercedes as well as of the judicial district. It is also the seat of the Roman Catholic Bishop of Mercedes and Luján.
Mercedes was first established as a fortress against native attacks. Its original name was "La Guardia de Luján" and it was one of several fortress built in the borders of Buenos Aires to protect this city and gather the people living in the county near.
It became a town on 25 June 1752 when founded by José de Zarate during a military campaign known as "La Valerosa". In 1777 viceroy Pedro de Ceballos proposed moving the town, but actually it was moved to its present location by viceroy Juan José de Vértiz on 8 May 1779. When moved its name was changed to "Nuestra Señora de las Mercedes".