Mercedes is a city in Buenos Aires Province, Argentina. It is located 100 km west from Buenos Aires and 30 km southwest of Luján. It is the administrative headquarters for the district (partido) of Mercedes as well as of the judicial district. The Catedral Basílica de Mercedes-Luján, located in the city, is the seat of the Roman Catholic Archdiocese of Mercedes-Luján.

The Archdiocese of Mercedes-Luján is a Latin Church ecclesiastical territory or archdiocese of the Catholic Church in north-central Buenos Aires Province, Argentina. It was immediately subject to the Holy See, coming into 2019. Erected as the Diocese of Mercedes on 20 April 1934, it was renamed as the Diocese of Mercedes-Luján on 10 May 1989 and elevated to its status as an archdiocese on 21 November 1997. Its mother church is the Catedral Basílica de Mercedes-Luján in Mercedes.