Erected by Pope Pius X, March 1, 1911, and committed to the Order of Friars Minor Capuchin, extended to all the Marianas Islands, July 4, 1946, Wake Island, June 14, 1948.

The Diocese of Agaña was established on October 14, 1965, as a suffragan of the Archdiocese of San Francisco, California. Elevated to Metropolitan Archdiocese, May 20, 1984, with suffragan Sees of the Diocese of the Caroline Marshalls and Diocese of Chalan Kanoa (subsequently added January 13, 1985). Member of the CEPAC Conference, Member of the Federation of Catholic Bishops of Conference of Oceania. Observer to NCCB-USCCB. Guam is an unincorporated Territory of the U.S.A. by an Act of U.S. Congress July 21, 1949. Legal Title: Archbishop of Agaña, a Corporation Sole.

The territory of the archdiocese comprises the island of Guam.  The island of Guam, the largest and southernmost of the Marianas Islands located in the western Pacific, is 209 square miles and has an estimated population of 166,445.