Un giorno di regno, ossia il finto Stanislao (A One-Day Reign, or The Pretend Stanislaus, but often translated into English as King for a Day) is an operatic melodramma giocoso in two acts by Giuseppe Verdi to an Italian libretto written in 1818 for the Bohemian composer Adalbert Gyrowetz by Felice Romani, based on the play Le faux Stanislas written by the Frenchman Alexandre Vincent Pineu-Duval in 1808. Un giorno was given its premiere performance at the Teatro alla Scala, Milan on 5 September 1840.

After the success of his first opera, Oberto in 1839, Verdi received a commission from La Scala impresario Merelli to write three more operas. Un giorno was first of the three, but he wrote the piece during a period when first his children and then his wife died and its failure in 1840 caused the young composer to almost abandon opera. It was not until he was enticed to write the music for the existing libretto of what became Nabucco that Verdi restarted his career.