At the foot of the old fortress, Grad, an inscription was found in Latin where it is possible to discern that the fortress was probably called Osinium in the antique period and was the residence of the Illyrian tribe the Osiniums.
From the end of the 13th century, Grad in Sinj was in the hands of the powerful Bribir dukes, Šubić, and during the 14th century fell under the rule of the Nelipčić dukes. It was conquered by the Ottomans in 1515, and after a few unsuccessful attempts, the Venetians captured Sinj’s fortress again in September 1686. In August 1715, the Ottomans tried to to conquer Sinj and held the town under a long-lasting siege, however, during the night between the 14th and the 15th August they retreated and ran away leaving around 10,000 dead and much destruction. The inhabitants of Sinj attribute this victory to the miraculous intervention of Our lady of Sinj, whose picture, hung in the fortress throughout the siege. The climb towards the Grad in Sinj fortress, starts from King Tomislav square, and continues through Kačić street, through Dalbel passage between the cypresses coming out in front of the votive church of our lady of Sinj.