During the Middle Ages, Omiš was notorious for the Omis Pirates, who, using specially built ships called the "Sagittae" (Arrow), ruled the Adriatic Sea for over three hundred years.
Despite numerous war alliances against Omis, including even the crusade against Omiš declared by the Pope himself, it was not until 1444 that Venice succeeded in ending centuries of Omis' domination of the Adriatic. Venice succeeded thanks to the growing Turkish threat and the rule of the whole of Dalmatia, except for the cities of Dubrovnik and Omiš. The danger from the Turks was growing, but the Turks never conquered Omiš.
Due to constant threats from land and sea, the fortresses of Starigrad (Fortica), Peovica (Mirabela) and Viseć were built, and the city is surrounded by high walls. In the Priko area is the most important monument in Omis, the Church of St. Peter, built in the 10th century.