Bartolo suspects that the drunken soldier is an acquaintance of Count Almaviva. The Count reappears, disguised as “Don Alonso,” a music teacher substituting for the supposedly ailing Don Basilio. He produces Rosina’s note to Lindoro, saying he found it at the inn where the Count is staying. He pleases Bartolo by offering to make Rosina believe the Count is deceiving her. Rosina recognizes Don Alonso as Lindoro and uses her music lesson to express her love for him. Figaro appears, ready to shave Bartolo. He takes the old man’s keys to the linen closet, using this opportunity to steal the balcony key. When Don Basilio appears, the Count signals Bartolo about their scheme to deceive Rosina. He makes clear that Basilio must be disposed of immediately. Convinced by everyone that he is terribly ill, Basilio accepts a purse from Almaviva and departs. While Figaro shaves Bartolo, Rosina and “Alonso” plan their elopement, but Bartolo finally realizes that he has again been tricked.
Rosina is miserable when Bartolo shows her own note, convincing her that Lindoro is plotting to give her up to the Count. She offers to marry Bartolo, revealing her previous elopement plans. No sooner has Bartolo gone after the police and the notary than Figaro and the Count appear, having used the stolen key. Rosina reproaches the Count, but is delighted when he reveals his true identity. Bartolo rushes in too late to stop the wedding. He accepts the situation and gives the couple his blessing.