Regina Diaz is an opera in two acts composed by Umberto Giordano to a libretto by Giovanni Targioni-Tozzetti and Guido Menasci. It premiered on 5 March 1894 at the Teatro Mercadante in Naples. The libretto is based on Lockroy's and Edmond Badon's Un duel sous le cardinal de Richelieu, which was also the source of Donizetti's 1843 opera Maria di Rohan, although the setting for Giordano's version was moved from 17th-century Paris to 18th-century Naples. The opera was a failure at its premiere and withdrawn after the second performance. Giordano's patron and publisher, Edoardo Sonzogno, blamed the failure on the poor libretto. Giordano blamed it on Sonzogno's interference in the production.