The 1866 Treaty of Vienna was an agreement signed on 3 October 1866 and ratified on 12 October by the Kingdom of Italy and the Austrian Empire that concluded the hostilities of the Third War of Italian Independence, a theatre of the concurrent Austro-Prussian War.

The Third Italian War of Independence was a war between the Kingdom of Italy and the Austrian Empire fought between June and August 1866. The conflict paralleled the Austro-Prussian War and resulted in Austria conceding the region of Venetia to France, which was later annexed by Italy after a plebiscite. Italy's acquisition of this wealthy and populous territory represented a major step in the Unification of Italy.