According to the Treaty of Vienna signed on 12 October 1866, the Austrian Empire ceded Venetia to the French Empire, which in turn would cede it to the Kingdom of Italy, «under the reservation of the "consent of the people duly consulted"». This represented the final division of the Kingdom of Lombardy-Venetia, as Lombardy had been ceded to the Kingdom of Sardinia by the Treaty of Zurich in 1859.
References
Treaty of Vienna (1866) Wikipedia(Text) CC BY-SA