Figaro and Susanna, Count Almaviva's servants, are preparing for their imminent wedding (“Cinque, dieci”), when Susanna reveals that she is being pursued by the count. Figaro is livid and plans to outwit the Count (“Se vuol ballare, signor Contino”).
Meanwhile Bartolo and Marcellina are trying to hold Figaro to a promise he made to marry Marcellina if he failed to pay back a loan to her (“La vendetta, oh la vendetta”).
The page Cherubino after describing his emerging infatuation with all women (“Non so più cosa son, cosa faccio”) tells Susanna of his love for the countess, just as the count arrives. Cherubino hides and overhears Almaviva's attempted seduction of Susanna. Don Basilio's arrival forces the count into hiding as well, which, in turn, forces Cherubino to move from his hiding place. The indiscreet Basilio talks of the count's love for Susanna and of Cherubino's love for the countess. The count then emerges from hiding, precipitating a scene that ends with Cherubino being packed off by the count to join the army. Figaro gives Cherubino mocking advice about his new, harsh, military life (“Non più andrai”).
The countess, distressed by her husband's philandering (“Porgi amor”), plots with Susanna to humiliate him: Susanna will write to the count, agreeing to a rendezvous, but they will send Cherubino in her place, dressed in women's clothes. Susanna urges Cherubino to sing the song he wrote for the Countess (“Voi che sapete”), then begins to undress him (“Venite, inginocchiatevi”). As Cherubino is being disguised , the count arrives, and Cherubino is pushed into an adjoining room. The count, hearing noises, leaves to fetch tools with which to break open the door. In the meantime Cherubino leaps out of the window while Susanna takes his place. When the count returns and finds that it is Susanna in the room, he is forced to apologize for his suspicions.To add to the confusion Figaro then arrives, then an irate gardener (Cherubino's leap has destroyed some plants), and finally Basilio and Bartolo with Marcellina, who wants her claim on Figaro honoured (“Voi signor, che giusto siete”).
At the urging of the Countess, Susanna gives a false promise to meet the Count later that night in the garden (“Crudel! Perché finora”). As Susanna leaves, the Count overhears her telling Figaro that he has already won the case. Realizing that he is being tricked, he resolves to punish Figaro by forcing him to marry Marcellina (“Vedrò mentr'io sospiro”). Figaro stalls Marcellina by telling her that he is of noble birth and cannot marry without his parents' consent. He reveals a birthmark on his arm, whereupon Marcellina realizes that she and Bartolo have found their long-lost son.
The Countess, alone, ponders the loss of her happiness (“Dove sono i bei momenti”). The countess dictates the note that Susanna is to pass to the Count, (“Canzonetta sull' aria”): the letter instructs the Count to return the pin which fastens the letter. A chorus of young peasants, among them Cherubino disguised as a girl, arrives to serenade the Countess. The Count arrives with Antonio and, discovering the page, is enraged. His anger is quickly dispelled by Barbarina. A double wedding is prepared, Susanna and Figaro being joined by Marcellina and Bartolo.
Following the directions in the letter, the Count has sent the pin back to Susanna, giving it to Barbarina. Unfortunately, Barbarina has lost it (“L'ho perduta”). Barbarina inadvertently reveals to Figaro that Susanna has sent a message to the count. Marcellina urges caution (“Il capro e la capretta”) but Figaro assumes the worst, and decides to catch his errant wife when she arrives for her tryst in the garden (“Aprite un po' quegli occhi”), and tells Bartolo and Basilio to come to his aid when he gives the signal. Susanna waits in the garden (“Deh vieni, non tardar”). The Countess arrives in Susanna's dress. Cherubino shows up and starts teasing her, but the Count gets rid of him by striking out in the dark. His punch actually ends up hitting Figaro. The real Susanna enters, now wearing the Countess' clothes. Figaro mistakes her for the real Countess, and starts to tell her of the Count's intentions, but he suddenly recognizes his bride in disguise until Susanna, fooled, loses her temper and slaps him. Figaro finally lets on that he has recognized Susanna's voice, and they make peace ("Pace, pace, mio dolce tesoro"). The Count hears Figaro declaring his love for the Countess and calls for his people. Bartolo, Basilio and Antonio enter with torches as, one by one, the Count drags out Cherubino, Barbarina, Marcellina and the "Countess" from behind the pavilion. All beg him to forgive Figaro and the "Countess", but he loudly refuses, until finally the real Countess re-enters and reveals her true identity. The Count, ashamed and remorseful, kneels and pleads for forgiveness himself. The Countess forgives her husband and all are contented. The opera ends in universal celebration.