Gradisca d'Isonzo (Friulian: Gardiscja or Gardiscje; Slovene: Gradišče ob Soči; archaic German: Gradis am Sontig) is a town and comune (municipality) in the Regional decentralization entity of Gorizia in Friuli-Venezia Giulia, north-eastern Italy.

The strategically-important area on the Isonzo River was probably already settled in Roman times and under the Kingdom of the Lombards, later exposed to the attacks by Hungarian forces on Northern Italy.

The Venetians fortified Gradisca as a bastion against Ottoman raids, then part of a massive defence line along the Isonzo River relying on plans designed by Leonardo da Vinci.

During the War of the League of Cambrai against Venice, Gradisca was conquered by Emperor Maximilian I in 1511, and thenceforth it was a Habsburg possession ruled within the County of Gorizia as part of the Inner Austrian lands.

The town was however kept by the Imperial House of Habsburg; in 1647 Emperor Ferdinand III made it the capital of an autonomous County of Gradisca and sold the territory to his confident John Anthony of Eggenberg.