Scene 1: The ruined city of Aquileia
Attila and his victorious horde are surprised to see a group of women spared as prisoners of war. Their leader, Odabella, asks why the Huns' women remain at home (Allor che i forti corrono / "While your warriors rush to their swords like lions"). Attila, impressed by her courage, offers a boon and she asks for her sword to avenge the death of her father at Attila's own hand (Da te questo or m'è concesso / "O sublime, divine justice by thee is this now granted"). The Roman envoy Ezio asks for an audience and proposes a division of the empire: Avrai tu l'universo, Resti l'Italia a me / "You may have the universe, but let Italy remain mine". Attila denounces him as a traitor to his country.
Scene 2: A swamp, the future site of Venice
A boat bearing Foresto and other survivors arrives; he thinks of the captive Odabella (Ella in poter del barbaro / "She is in the barbarian's power!") but then rouses himself and the others to begin building a new city (Cara patria già madre e reina / "Dear homeland, at once mother and queen of powerful, generous sons").
Scene 1: A wood near Attila's camp
Odabela laments her father and Foresto (Oh! Nel fuggente nuvolo / "O father, is your image not imprinted on the fleeting clouds?...") believing the latter to be dead. When he appears, she is put on the defensive, denying any infidelity and reminding him of the biblical Judith. The couple is reunited: Oh, t'inebria nell'amplesso / "O vast joy without measure")
Scene 2: Attila's tent
Attila awakes and tells Uldino of a dream in which an old man stopped him at the gates of Rome and warned him to turn back (Mentre gonfiarsi l'anima parea / "As my soul seemed to swell"). In the daylight, his courage returns and he orders a march (Oltre quel limite, t'attendo, o spettro / "Beyond that boundary I await you, O ghost!"). However, when a procession of maidens clad in white approaches, singing a Christian hymn, he recognizes the Roman bishop Leo as the old man of his dream, and collapses in terror.
Ezio has been recalled, after a peace has been concluded. He contrasts Rome's past glory with the child emperor Valentine (Dagl'immortali vertici / "From the splendid immortal peaks of former glory"). Recognizing the incognito Foresto among the bearers of an invitation to a banquet with Attila, he agrees to join forces (E' gettata la mia sorte / "My lot is cast, I am prepared for any warfare" ). At the banquet, Foresto's plot to have Uldino poison Attila is foiled by Odabella, jealous of her own revenge. A grateful (and unsuspecting) Attila declares she shall be his wife, and places the unmasked Foresto in her custody.
Uldino informs Foresto about the plans for the wedding of Odabella and Attila; Foresto laments Odabella's apparent betrayal (Che non avrebbe il misero / "What would that wretched man not have offered for Odabella). Ezio arrives with a plan to ambush the Huns; when Odabella comes Foresto accuses her of treachery, but she pleads for his trust. Attila finds the three and recognizes their betrayal. As Roman soldiers approach, Odabella stabs him with the sword he had given her. The three conspirators cry that the people have been avenged.