Carlos seeks peace at the monastery of St. Just in Spain, where he prays at the tomb of his grandfather, Emperor Charles V. A monk, who seems to be the emperor’s ghost, confronts him. His friend Rodrigue, the Marquis of Posa, arrives to remind Carlos of his commitment to the cause of the Flemish people, who are oppressed by Spanish rule. When Philippe and Élisabeth arrive, Carlos confesses to Posa his love for the queen, and the two men pledge themselves to the cause of liberty and swear eternal friendship.

In a garden outside the monastery, Princess Eboli entertains the other ladies of the court with a song. Élisabeth enters, followed by Posa, who hands her a secret letter from Carlos asking for a meeting. When he is admitted, Carlos asks the queen to obtain Philippe’s permission for him to go to Flanders, then suddenly declares his continuing love. Élisabeth rejects him, and Carlos rushes off. The king enters and, finding the queen alone, banishes the Countess of Aremberg, who should have been attending her. Left alone with the king, Posa challenges Philippe to end his oppression of the Flemish people. Philippe refuses but is impressed by Posa’s courage. He warns him to beware of the Inquisition and tells Posa that he suspects his wife and Carlos, asking Posa to watch them. Posa accepts the assignment, knowing that being in the king’s confidence will be valuable.