- 1790 - Opera buffa Despina urges them to succumb to the Albanians' overtures. After she leaves, Dorabella confesses to Fiordiligi that she is tempted, and the two agree that a mere flirtation will do no harm and will help them pass the time while they wait for their lovers to return.
Scene 2: The garden
Dorabella and the disguised Guglielmo pair off, as do the other two. The conversation is haltingly uncomfortable, and Ferrando departs with Fiordiligi. Now alone, Guglielmo attempts to woo Dorabella. She does not resist strongly, and soon she has given him a medallion (with Ferrando's portrait inside) in exchange for a heart-shaped locket. Ferrando is less successful with Fiordiligi, so he is enraged when he later finds out from Guglielmo that the medallion with his portrait has been so quickly given away to a new lover. Guglielmo at first sympathises with Ferrando but then gloats, because his betrothed is faithful.
Scene 3: The sister's room
Dorabella admits her indiscretion to Fiordiligi. Fiordiligi, upset by this development, decides to go to the army and find her betrothed. Before she can leave, though, Ferrando arrives and continues his attempted seduction. Fiordiligi finally succumbs and falls into his arms. Guglielmo is distraught while Ferrando turns Guglielmo's earlier gloating back on him. Alfonso, winner of the wager, tells the men to forgive their fiancées. After all: Così fan tutte—"All women are like that."
Scene 4
It begins as a double wedding for the sisters and their Albanian grooms. Despina, in disguise as a notary, presents the marriage contract, which all sign. Directly thereafter, military music is heard in the distance, indicating the return of the officers. Alfonso confirms the sisters' fears: Ferrando and Guglielmo are on their way to the house. The Albanians hurry off to hide (actually, to change out of their disguises). They return as the officers, professing their love. Alfonso drops the marriage contract in front of the officers, and, when they read it, they become enraged. They then depart and return moments later, half in Albanian disguise, half as officers. Despina has been revealed to be the notary, and the sisters realize they have been duped. All is ultimately forgiven, as the entire group praises the ability to accept life's unavoidable good times and bad times.