Italy's longest international border is with which country?

Italy's longest land border is with Switzerland.

The border between the modern states of Switzerland and Italy extends to 744 km, from the French-Swiss-Italian tripoint at Mont Dolent in the west to the Austrian-Swiss-Italian tripoint near Piz Lad in the east. Much of the border runs across the High Alps, rising above 4,600 meters as it passes east of Dufourspitze, but it also descends to the lowest point in Switzerland as it passes Lago Maggiore below 200 meters.

The border is a product of the Napoleonic period, established with the provisional constitution of the Helvetic Republic of 15 January 1798, restored in 1815. While this border existed as a border of Switzerland from 1815, there was only a unified Italian state to allow the existence of a "Swiss-Italian border" with the formation of the Kingdom of Italy in 1861, it previously comprised the borders between Switzerland and the Kingdom of Sardinia, the Kingdom of Lombardy–Venetia and the province of Cisleithania of Austria-Hungary.