Un Ballo in Maschera, or A Masked Ball, is an opera in three acts by Giuseppe Verdi with text by Antonio Somma. It is based on the libretto Gustave III by Eugène Scribe (originally set by Daniel Auber), who was inspired by events surrounding the assassination of King Gustav III of Sweden at a masquerade ball in 1792. The opera's first production was at the Teatro Apollo, Rome in 1859.

Un Ballo in Maschera ended up being one of Verdi's most controversial operas. During composition, the censors required extensive changes, eventually demanding more alterations than Verdi was willing to make. Therefore, he broke his contract and was sued by the theater, which he then counter-sued for libel. Eventually, the legal fight ended with withdrawal of the theater's charges and Verdi promising to finish the work. However, prior to its premier in Rome, the scene was forced to change from Stockholm to Sweden or Boston, Massachusetts.

ACT I. The Count of Warwick, Riccardo, opens a hearing attended by his enemies, who plot to murder him at a ball. A judge threatens to banish Ulrica, a fortune teller, but the count proposes that those present visit her home. Here, she tells several fortunes, including a secret fortune for Amelia, who is the secret love of Riccardo. Finally, she foretells a disguised count of his own death by a friend.

ACT II. Amelia looks for a magic herb described to her by Ulrica and is followed by Riccardo, who professes his love. She is unsure what to do, because while she also loves him, she is already married. Renato, Amelia's husband and Riccardo's secretary, arrives and tells Riccardo to leave for his safety. Riccardo leaves, but leaves the disguised woman with Renato. Then, the conspirators Samuel and Tom arrive and in the struggle with Renato, Amelia's veil falls. Shocked, Renato agrees to meet the conspirators the next day.

ACT III. Renato decides to avenge his anger by killing Riccardo instead of his wife, after she asks to see their son one more time. Samuel and Tom arrive, and doubt Renato's word until he offers his son as a guarantee. Then, Renato's name is drawn by Amelia in a lottery to determine who shall be the murderer. After being invited to the ball, Amelia tries to think of a way to save her love while the others decide to use the ball as the scene of the murder. Meanwhile, Riccardo decides to renounce Amelia. Finally, at the ball, Renato discovers which mask hides Riccardo, and so stabs him with a dagger. Prior to Riccardo's death, he pardons the conspirators and reveals that Amelia was never unfaithful.