The Diocese of Senj-Modruš (German: Zengg-Modrus, Italian: Diocesi di Modruš, Latin: Dioecesis Modrussensis) was located in the historical Kingdom of Croatia, while it was in personal union with Kingdom of Hungary, and it was suffragan of Zagreb.
The See of Modruš (Modrus) was established at Krbava (Latin: Corbavia) in Lika region in 1185.
Pius II (1458-1464) moved the former see from Krbava to Modruš, as it suffered from the advance of the Turks.
The diocese consisted of Rijeka, of some parts of the "Komitat" of Zagreb, and of the Croatian Military Frontier.
The language at Mass and during the services was Old Slavic, by reason of a papal privilege.