Iago continues to use his words as weapons to break down Othello. He also uses the motif of appearance vs reality in his favor. His psychological torture is too much for Othello and he falls into a trance (he has a seizure). Iago tells Othello to hide while he talks to Cassio and gets him to admit he is having an affair with Desdemona. He promises to get Cassio to admit the time, the place, and how often they've been together. Iago gets very lucky indeed. Cassio comes down and talks about Bianca while Iago speaks softly to him to serve his purpose, and loudly when he wants Othello to hear. It makes it seem as though he is talking about being with Desdemona. Luckily for Iago, Bianca comes in with the handkerchief and throws it at Cassio; angry that he gave her a gift that belonged to another woman. (He actually found it after Iago planted it in his room). Of course, Cassio has no idea what she is talking about, and he follows her off stage. Othello's first remark is, "How shall we murder him, Iago?"

Scene ii: Othello tries to get Emilia to tell him what he is now convinced of; that Desdemona is cheating. He then treats and talks to Desdemona as though she were a whore. Desdemona is shocked and upset by Othello's treatment of her. Emilia is outraged. She thinks that some villain has been pouring poison in Othello's ear; ironically it is her husband that is guilty of this crime. Iago assures Desdemona that Othello is only upset by some problem with affairs of state and comforts her as a good friend might. Desdemona, like the other characters, trust 'Honest Iago."

Roderigo complains to Iago that he has gotten nothing from all his efforts, and threatens to quit his pursuit of Desdemona. He is very angry with Iago, but Iago persuades him that he will bed Desdemona within two nights if he would just murder Cassio. Iago enjoys being about to manipulate people.

Scene iii: Othello tells Desdemona to dismiss Emilia, and go to bed to wait for him. She asks Emelia to put their wedding sheets on the bed before she goes. Desdemona gets ready for bed and sings "Willow," a song of lost love. This is foreshadows her death. Desdemona asks Emilia if there really are women who commit adultery; Emilia tells her that there are, but their sins are the fault of their husbands. Desdemona she cannot imagine such a woman. This conversation illuminates her innocence and causes pity in the audience.