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).
The Senj Catholic diocese was established in 1169. Croatian-Hungarian king B�la III gave the town to the Templars in 1184, and in 1271 it became the property of the counts of Krk, the Frankopans.
In 1248 the bishop of Senj was allowed by the Pope Innocent IV to use the Glagolitic alphabet and the vernacular language in liturgy. A Glagolitic printing press was set up in 1494, and produced the incunabula "The Glagolic Missal" and "Spovid op?ena".
The military captaincy of Senj was established in 1469 in order to defend from the invading Ottoman and Venetian armies, and the town sheltered thousands of refugees from the nearby occupied areas. The fortress Nehaj was completed on the hill Nehaj in 1558 that in that time was beside the town. This hill is today in the towns borders. The wars with the Ottomans lasted well into the 17th century. In that time in Senj lived uskoci, that where a tribe that dwelled in the fortress Nehaj. For the wars with the Ottomans they were important because they were small units of men rowing swift boats, and so they proved to be very efficient guerrilla forces.
The 18th century brought some prosperity, especially with the construction of the Josephina (named after Emperor Joseph II) linking the Adriatic coast via Senj to Karlovac. The railway line built in 1873 between Fiume (Rijeka) and Karlovac didn't go past Senj and this deterred its further development.
In the fall of 1943, during World War II, when Fascist Italy capitulated, the Partisans took control of Senj and used it as a supply port. Subsequently, the Luftwaffe started bombarding the city
