A street in Seville. Night time.
Under the windows of Doctor Bartolo's house the young Count of Almaviva serenades the beautiful Rosina, the woman he is in love with and who is Doctor Bartolo's wealthy ward.
Figaro, the famous barber of Seville, appears. In a conversation with him Count Almaviva finds out that Doctor Bartolo is soon intending to marry Rosina himself.
Rosina appears on the balcony with a message in her hand. Doctor Bartolo, as always jealous and mistrustful, is watchfully by her side. The message that Rosina holds provokes his suspicions.

Rosina tries to explain everything, but to no avail, as nothing can convince Bartolo that his future bride is loyal. He decides to rush to the notary and hurry the marriage along.
Now Rosina is alone and the Count has the opportunity to speak to her. He presents himself as Lindoro, a poor student, so that Rosina will not be influenced by his rank. The girl is favourable towards "Lindoro" and a feeling of love is kindled within her. Figaro and Almaviva make a plan how to get in to the Doctor's house. The barber suggests Almaviva dress himself as a soldier and demand to be quartered in Bartolo's house.

A room in Bartolo's house.
Rosina writes a love letter to "Lindoro".
Don Basilio, Rosina's teacher of music, arrives. He tells Bartolo that there are some rumours in the town that the Count of Almaviva is in love with Rosina and is seeking access to her. To please his friend, Don Basilio offers to spread some gossip around about Almaviva, and in such a way help Bartolo to eliminate his fellow suitor. Figaro, however, overhears the conversation of the two plotters and tells it all to Rosina.
Almaviva arrives, dressed as a soldier. He pretends to be a little drunk. Bartolo refuses to have him in his house, stating that he is exempt from housing the military. A quarrel starts.

A police officer and his soldiers arrive, trying to resolve the situation. They are to arrest the brawler, but the Count whispers his name to the officer and is released, to everyone's astonishment.

Almaviva appears again in Bartolo's house. This time dressed as a teacher of music, he announces that Don Basilio is ill and he, "Don Alonso", has been sent to replace the master. Bartolo believes the new teacher looks a bit suspicious. To gain Bartolo's confidence, "Don Alonso" produces a note, written by Rosina, which he says proves the Count is making a fool of Rosina. Delighted by the prospect of an effective libel, Bartolo admits Don Alonso and summons Rosina, who can hardly hide her amazement in seeing "Lindoro". Nevertheless, Figaro arrives to shave Bartolo and Almaviva gets the chance to talk to Rosina in private.
Suddenly Don Basilio arrives. His arrival is indeed a big threat to the plans of the two lovers. Figaro, "Lindoro" and Rosina beg Basilio to go back home, as in their opinion he doesn't really look in good health, even worse-it might be scarlet fever! Don Basilio is given a purse of gold, and without understanding the situation, he leaves. Bartolo, though, is now listening to the conversation of "Don Alonso" and Rosina and understands that his ward is about to run away with her beloved. Bartolo appears and, desperate to stop the girl, he manages to convince Rosina that "Lindoro" actually will hand her over to another man-the Count of Almaviva.
A storm breaks. Figaro and Almaviva arrive, everything is now ready for Rosina's escape. Figaro has managed to steal the keys for the balcony from Doctor Bartolo. Rosina is now confused and fearful and refuses to leave. In the end it turns out that Lindoro and Almaviva are one and the same person. Rosina is ecstatic.
Don Basilio and a notary arrive. They have been invited by Doctor Bartolo to the registration of the marital contract between him and Rosina.

But the Doctor is not at home. Figaro, ready with an idea as always, announces to the notary that he actually has been invited to register the marriage between Almaviva and Rosina. Don Basilio, with the help of another purse of gold, signs as a witness. Doctor Bartolo arrives and despairs at what has happened. All his efforts to have a young, rich and pretty wife have been in vain! There is nothing left for him but to accept defeat and bless the happy couple.