The Concordat of 1851 was a concordat between the Spanish government of Queen Isabella II and the Vatican. It was negotiated in response to the policies of the anticlerical Liberal government, which had forced her mother out as regent in 1841. Although the concordat was signed on 16 March 1851, its terms were not implemented until 1855. (A second concordat was negotiated in 1859, as a supplement to the Concordat of 1851.)