Tosca is an Italian opera in three acts by Giacomo Puccini , first performed in Rome , on January 14 , 1900 , libretto by Giuseppe Giacosa and Luigi Illica , after Victorien Sardou 's play La Tosca . Tosca is an opera in three acts by Giacomo Puccini to an Italian libretto by Luigi Illica and Giuseppe Giacosa . It premiered at the Teatro Costanzi in Rome on 14 January 1900 .