Act II
By the castle garden, Iago advises Cassio to seek Desdemona's aid in regaining Otello's favor. When the grateful Cassio goes off, Iago professes his belief in a cruel god, in whose image man was created ("Credo"). On Otello's arrival, the ensign calls attention to Cassio, taking his leave of Desdemona and Emilia, Iago's wife, in the garden. Iago makes subtle innuendos about Desdemona's fidelity, and then warns the Moor to beware of jealousy. Women, children, and sailors bring flowers to Desdemona, whose beauty softens Otello's newly aroused suspicions, but when she begins to plead for Cassio's reinstatement, he grows irritable. She tries to bind his brow with a handkerchief, but he throws it to the ground. As the uncomprehending Desdemona declares her devotion, Iago furtively wrests the handkerchief from Emilia, who has retrieved it. When the women leave, Otello accuses his ensign of destroying his peace of mind. Iago answers the Moor's demands for proof by claiming he has heard Cassio murmur Desdemona's name in his sleep ("Era la notte"); worse, he says he has seen in Cassio's hand the embroidered handkerchief Otello gave her when he first courted her. Seconded by Iago, Otello vows vengeance ("Si, pel ciel").

Act III
In the armory, Iago promises Otello more proof, then departs as Desdemona greets her husband ("Dio ti giocondi). The Moor hints at his suspicions, but she fails to understand. When he demands the handkerchief, she again pleads for Cassio. Otello, sure of her guilt, calls her a courtesan. She tearfully declares her innocence, but the Moor sends her away. His rage spent, he reflects brokenly that he could have endured any affliction but this ("Dio! Mi potevi scagliar"), then hides as Cassio and Iago approach. The latter, flashing the handkerchief, manipulates Cassio's banter about his mistress so that Otello thinks he means Desdemona. Cassio leaves as trumpets announce dignitaries from Venice. Otello resolves to kill his wife.

In the great hall, the court enters to welcome Lodovico, the ambassador, who brings orders recalling Otello to Venice and naming Cassio governor. Losing his self-control, Otello hurls Desdemona to the floor. She begs forgiveness for her supposed crime. As the stunned courtiers try to console her, Otello orders them out. The Moor falls in a fit, and Iago ironically salutes him as the Lion of Venice.