Act III begins with Marcus and Portius talking in a chamber. Portius insists on their friendship having “severest virtue for its basis,” and Marcus asks him to “indulge me but in love,” bringing up his anguish over his love for Lucia (33). Marcus begs Portius to tell Lucia of his love for her, but Portius refuses and feels conflicted because of his own feelings. “If I disclose my passion, / Our friendship’s at an end: if I conceal it, / The world will call me false to a friend and brother,” he says in an aside (34). Lucia enters after Marcus leaves, and she and Portius talk of Marcus’s love. Lucia explains that she and Portius cannot be together because she sees “thy sister’s tears, / Thy father’s anguish, and thy brother’s death, / In the pursuit of our ill-fated love” (35). Lucia bids Portius goodbye, and though he fights back in an attempt to still have her, he admits he “must approve the sentence that destroys me” (36).

After Lucia leaves, Marcus enters, and Portius tells him that Lucia “compassionates your pains, and pities you,” rather than loves him (37). Marcus lashes out at Portius in anger, though he quickly apologizes for his temper.