| Type: | Bishop |
| Honorific-Prefix: | Most Reverend |
| Diego Castejón Fonseca | |
| Bishop of Tarazona | |
| Church: | Catholic Church |
| Diocese: | Diocese of Tarazona |
| Term: | 1643–1655 |
| Predecessor: | Baltasar Navarro Arroytia |
| Successor: | Pedro Manero |
| Consecration: | 2 July 1634 |
| Consecrated By: | Melchor Soria Vera |
| Birth Date: | 1580 |
| Birth Place: | Madrid, Spain |
| Death Date: | 19 February 1655 (age 75) |
| Death Place: | Tarazona, Spain |
| Previous Post: | Bishop of Lugo (1634–1636) |
| Nationality: | Spanish |
Diego Castejón Fonseca (1580 – 19 February 1655) was a Roman Catholic prelate who served as Bishop of Tarazona (1643–1655) and Bishop of Lugo (1634–1636).[1]
Diego Castejón Fonseca was born in Madrid, Spain in 1580. On 9 January 1634, he was appointed during the papacy of Pope Urban VIII as Bishop of Lugo. On 2 July 1634, he was consecrated bishop by Melchor Soria Vera, Titular Bishop of Troas, with Miguel Avellán, Titular Bishop of Siriensis, and Timoteo Pérez Vargas, Bishop of Ispahan, serving as co-consecrators. On 1 May 1636, he resigned as Bishop of Lugo.
In June 1640, he became President of the Council of Castile.On 28 March 1643, he was selected by the King of Spain and confirmed on 23 May 1644 by Pope Urban VIII as Bishop of Tarazona. He served as Bishop of Tarazona until his death on 19 February 1655.