In 569 as Diocese of Magnetum (Latin) / Magneto (Curiate Italiano and Portuguese) / Magneten(sis) (Latin adjective) was established on territory of the Kingdom of the Suebi in Galicia, split off from the Archdiocese of Braga, its apparent Metropolitan.
In 585, when the kingdom was annexed by the Visigoths, and was turned into the sixth province of the Visigothic Kingdom of Hispania, the bishopric was suppressed, its territory being reassigned to establish the
Diocese of Portucale (present see of Porto).
Its only recorded residential bishop was :
The diocese was nominally restored in 1969 as Latin Catholic Titular bishopric of Magnetum (Latin) / Magneto (Curiate Italian) / Magneten(sis) (Latin adjective).
It has had the following incumbents, of the fitting Episcopal (lowest) rank with an archiepiscopal (current) exception :
- Angelo Frosi, Xaverian Missionary Fathers (S.X.) (1970.02.02 – 1978.05.26) ?Auxiliary? Bishop of Abaetetuba (Brazil) (1970.02.02 – 1981.09.17), Bishop of Abaetetuba (1981.09.17 – death 1995.06.28)
- Armido Gasparini, Comboni Missionaries of the Heart of Jesus (M.C.C.J.) (1979.03.15 – death 2004.10.21) as first Apostolic Vicar of Awasa (Ethiopia) (1979.03.15 – 1993.12.20); previously last Apostolic Prefect of Awasa (Ethiopia) (1973.02.16 – 1979.03.15)
- António Francisco dos Santos (2004.12.21 – 2006.09.21) as Auxiliary Bishop of Braga (Portugal) (2004.12.21 – 2006.09.21), later Bishop of Aveiro (Portugal) (2006.09.21 – 2014.02.21), Bishop of Porto (Portugal) (2014.02.21 – ...)
- Titular Archbishop Léon Kalenga Badikebele (2008.03.01 – ...), papal diplomat : Apostolic Nuncio (ambassador) to Ghana (2008.03.01 – 2013.02.22), Apostolic Nuncio to El Salvador (2013.02.22 – ...) and Apostolic Nuncio to Belize (2013.04.13 – ...).