The ancient town of Tragurion (“island of goats”) was founded as a trading settlement by Greek colonists from the island of Vis (Zssa) in the 3rd century BC on an islet at the western end of the bay of Manios, in a strait between the mainland and one of the Adriatic islands, where there was already a small settlement. The Hellenistic town was enclosed by megalithic walls and its streets were laid out on a “Hippodamian” grid plan: the line of the ancient cardo maximus is that of the modem main street. The town flourished in the Roman period as an oppidum civium romanum, linked with the neighbouring cities of Salona, capital of the Roman province of Dalmatia, and Siculi, a colony for Roman military veterans. During the Late Roman period it was extended and refortified. Extensive Roman cemeteries have been discovered, outside the town, as was customary, and a basilica was erected in one of these in Late Roman times. Although it was not made a bishopric in the early Christian period, Trogir was endowed with two large aisled basilicas, sited where the latter-day Cathedral and Benedictine Church of St John the Baptist now stand.
In the second half of the 9th century Trogir became part of the Byzantine theme of Dalmatia, with its capital Zadar, and it was occupied by Venice at the end of the 10th century. Early medieval Trogir expanded to the south and new fortifications were constructed. At the beginning of the 12th century Trogir accepted Hungarian rule when the theme of Dalmatia was overrun. There was a short period of Venetian rule in the early 14th century, but it was not until 1420 that the town became part of the Venetian empire. Between the 13th and 15th centuries many new building took place, this period seeing the construction of the Cathedral and the Camerlengo fortress, a radical remodelling of the main square, and two campaigns of reconstruction and strengthening of the fortifications. The Treaty of Campoformio handed Trogir over to the Austro-Hungarian Empire, to which it belonged, apart from a short period under French rule (1806-lo), as part of the Illyrian Provinces until 1918.