Ezzelino III da Romano (25 April 1194, Tombolo – 7 October 1259) was an Italian feudal lord, a member of the Ezzelini family, in the March of Treviso (in modern Veneto).

According to the chronicler Rolandino of Padua, the young Ezzelino already showed a keen interest in siegecraft and acquired a hatred of the Este which would last his entire life.

However, he and his brother Alberico later changed sides, when it became apparent that the League favoured their enemies in the March, particularly the Este and the San Bonifacio (Sambonifacio).

Because Ezzelino and his Veronese allies, the Monticuli and Quattuorviginti, had gained control of Verona in early 1236, the emperor could bring reinforcements across the Alps, including 3000 German men-at-arms.

In 1236 Ezzelino married Selvaggia, Frederick's natural daughter who was thirteen years old at the time; conquered Verona and by treason Padua, seizing the position of podestà of that city.

After a failed pacification attempt by Frederick, as soon as the emperor departed Ezzelino attacked the Este, submitting Treviso - even though it was his brother's fief - Belluno and Feltre.

He had reconciled with his brother and allied himself with other seigneurs of the Veneto and Lombardy, attacking Padua, which resisted, and Brescia, which was instead sacked after an easy victory of his German knights over the crusaders' army.

Before Ezzelino, the seizing of political power in city-states throughout the Middle Ages, had been based on real or pretended inheritance claims or else were directed against infidels and the excommunicated; but with him, as the historian Jacob Burkhardt relates, "Here for the first time the attempt was openly made to found a throne by wholesale murder and endless barbarities, by the adoption in short, of any means with a view to nothing but the end pursued.