Senj is the oldest town on the upper Adriatic, and it was founded in the time before the Romans some 3000 years ago (Senia) on the hill Kuk. It was the center of the Illyrian tribe Iapydes. The current settlement is situated at the foot of the slopes Mala Kapela and Velebit.
The symbol of the town, the Fortress Nehaj (Croatian: Tvrđava Nehaj), that is found on the hill Nehaj, was completed in 1558, and has since then been the seat of the Uskoci until they were banished from the town in the 1617 after the Habsburg Monarchy made peace with the Republic of Venice. Senj is now found in the Lika-Senj County, and is also included in the Roman Catholic Diocese of Gospić-Senj, and the Roman Catholic Archdiocese of Rijeka within the Catholic Church.
Senj has been inhabited since the prehistoric times. A settlement called Attienities in today's Senj was mentioned in Greek documents dated 4th century BC. The Romans used Senia as a stronghold against the Illyrians in the 2nd century BC, and it was a thriving town in their province of Dalmatia. The Avars and the Croats settled Senj in the 7th century (AD).