| Jurisdiction: | Diocese |
| Ilagan | |
| Border: | Catholic |
| Latin: | Dioecesis Ilaganensis |
| Local: | Diócesis ti Ilagan Diyosesis ng Ilagan Diócesis de Ilagán |
| Coat: | Diocese of Ilagan.svg |
| Coat Size: | 150px |
| Country: |  Philippines |
| Territory: | Isabela |
| Province: | Tuguegarao |
| Metropolitan: | Tuguegarao |
| Area Km2: | 12556 |
| Population: | 1,741,750 |
| Population As Of: | 2021 |
| Catholics: | 1,368,520[1] |
| Parishes: | 38 |
| Denomination: | Roman Catholic |
| Established: | 31 January 1970 |
| Cathedral: | Cathedral of Saint Michael the Archangel (Gamu) |
| Cocathedral: | Saint Ferdinand Parish Church (Ilagan; former cathedral) |
| Patron Title: | Titular patrons |
| Priests: | 38[2] |
| Bishop: | David William Antonio |
| Metro Archbishop: | Ricardo Baccay |
| Vicar General: | Edmundo "June" Castaneda |
The Roman Catholic Diocese of Ilagan (Lat: Dioecesis Ilaganensis) is a diocese of the Latin Church of the Catholic Church in the Philippines. It was erected on 31 January 1970 from territory of the then Roman Catholic Diocese of Tuguegarao and it covers the entire civil Province of Isabela. It is a suffragan diocese of the Roman Catholic Archdiocese of Tuguegarao.
The diocese has had no jurisdictional changes. The seat of the diocese used to be located in Saint Ferdinand Parish Church in Ilagan City before being transferred to the Gamu Cathedral (Cathedral of Saint Michael the Archangel) in Gamu, Isabela, in 2013.[3]
David William V. Antonio was installed as the fifth Bishop of Ilagan on February 12, 2019.[4]
| 1. | Francisco Cruces | (4 March 1970 – 22 August 1973) | ||
| 2. | Miguel Purugganan | (21 January 1974 – 26 July 1999) | ||
| 3. | Sergio Utleg | (26 July 1999 – 13 November 2006) | ||
| 4. | Joseph Amangi Nacua | (10 June 2008 – 25 February 2017) | ||
| 5. | David William Antonio | (12 February 2019 – present) |