A drawing-room at Violetta’s mansion in Paris.
At the house of Violetta Valéry, a beautiful and famous Parisian courtesan protected by Baron Douphol, a sumptuous party is in progress (introduction: «Dell’invito trascorsa è già l’ora...»). Among the guests enters Gastone de Letorières, who introduces Violetta to a silent but passionate admirer of hers: Alfredo Germont. Baron Douphol is irritated by their conversation and refuses to announce the toast, which Alfredo now proposes instead (toast: «Libiamo ne’ lieti calici »).

The dancing begins but Violetta, seized by a fit of coughing, is unable to join the others in the salon and sits down, alone, to regain her strength. As she observes her pale face in the mirror, she notices the presence of Alfredo, who confesses that he has been in love with her for a year (short duet: «Un dì felice, eterea»).

At first she rejects his declaration, but then hands him a camellia, her favourite flower, suggesting that he come back to see her when the flower has withered: the next day. Overcome with joy, Alfredo abandons the festivity, while Violetta goes back to join the others. The party ends with the guests seeing in the first light of dawn (stretta in the introduction: «Si ridesta in ciel l’aurora»).

Alone now, Violetta catches herself dreaming of possible genuine love at last (cantabile: «Ah fors’è lui che l’anima»). But she quickly dismisses these fantasies: for her there can be no other destiny but luxury and the pleasures of society life (cabaletta: «Sempre libera degg’io»).