Scene 1: Temple of Baal, Babylon
Assur and Semiramide, former lovers, had plotted to murder the latter's husband, King Nino, 15 years before. Her son escaped death at that time and is now, as Arsace, a successful commander, his identity unknown to his mother.
He expresses his concern as to why he has been called back to Babylon, declares his love for Azema who loves him, and his unwillingness to support Assur in the latter's bid for the throne. (Aria: "Eccomi alfine in Babilonia... Ah! quel giorno ognor rammento"). The two men discuss the situation of Azema with Arsace reaffirming his love for Azema: (Aria: "Bella imago degli dei").
Scene 2: The Hanging Gardens
Having fallen in love with Arsace and believing that he loves her, Semiramide waits for his arrival: (Aria: "Bel raggio lusinghier"). Arsace arrives and tells her of his love for Azema, but she still believes that he really loves here: (Aria: "Serbami ognor sì fido"). Declaring him king and her consort results in King Nino's ghost warning of the crimes to be expiated and the high priest Oroe tells Arsace about the crimes committed by his mother and Assur ("Giuri ognuno, a' sommi Dei", Semiramide, Arsace, Idreno, Oroe and Assur").
Arsace's father's tomb
Assur continues to pressure Semiraminde to make him king while she threatens to reveal the crime (Duet: "Se la vita ancor t'è cara"). In the tomb, Arsace meets King Nino's murderers and he is told that Nino is his father. He accepts that he must avenge his father: (Aria: "In sì barbara sciagura"). His mood ranges widely from despair to heroic determination. As Semiramide learns of Arsace's real identity and becomes remorseful, Arsace swears filial loyalty, wishings to spare his mother: (Duet: "Ebben, a te... Giorno d'orrore... Madre, addio!"). Assur learns that the people have turned against him and he vows to kill Arsace, then goes into the tomb to find the young man: ("Deh ti ferma...Que numi furenti"). Along with Oroe, mother and son go into the tomb where Assur confronts them: ("L'usato ardir"). Assur, seeking to strike Arsace, injures Semiramide as she steps between them to stop the fight. Assur is arrested, Semiramide dies, and Arsace relunctantly accepts that he shall be King.