The Roman Catholic Archdiocese of Mercedes-Luján (erected 20 April 1934, as the Diocese of Mercedes) is in Argentina and is immediately subject to the Holy See. It was renamed as the Diocese of Mercedes-Luján on 10 May 1989 and elevated on 21 November 1997.
| Year | Along with | To form |
|---|---|---|
| 1947 | Archdiocese of La Plata | Diocese of San Nicolás de los Arroyos |
| 1957 | Diocese of Azul | Diocese of Nueve de Julio |
| 1957 | Diocese of Bahía Blanca | Diocese of Santa Rosa |