This takes place in a great hall which looks out on the harbour. Fiesco has been set free, but Paolo is condemned to death for leading the uprising against the Doge. Paolo tells Fiesco that he has poisoned Simon. Fiesco goes to Simon, who is now dying (the poison is working slowly). Simon recognizes his old enemy who he thought was dead. He is happy to tell him that Amelia is his long-lost granddaughter. Fiesco feels terribly sad and tells Simon about the poison. Gabriele and Amelia have just got married. They find her father and grandfather are now friends. Simon asks for Gabriele to be the next Doge when he dies. Fiesco goes to the window and announces to the crowd that Gabriele Adorno is now the new Doge and that Simon Boccanegra is dead.