Boris Godunov (Russian: Борис Годунов, Borís Godunóv) is an opera by Modest Mussorgsky (1839–1881). The work was composed between 1868 and 1869 in Saint Petersburg, Russia. It is Mussorgsky's only completed opera and is considered his masterpiece. Its subjects are the Russian ruler Boris Godunov, who reigned as Tsar (1598 to 1605) during the Time of Troubles, and his nemesis, the False Dmitriy (reigned 1605 to 1606). The Russian-language libretto was written by the composer, and is based on the "dramatic chronicle" Boris Godunov by Aleksandr Pushkin, and, in the Revised Version of 1872, on Nikolay Karamzin's History of the Russian State.

Boris Godunov, among major operas, shares with Giuseppe Verdi's Don Carlos (1867) the distinction of having the most complex creative history and the greatest wealth of alternative material. The composer created two versions—the Original Version of 1869, which was rejected for production by the Imperial Theatres, and the Revised Version of 1872, which received its first performance in 1874 in Saint Petersburg. These versions constitute two distinct ideological conceptions, not two variations of a single plan.

Boris Godunov has seldom been performed in either of the two forms left by the composer, frequently being subjected to cuts, recomposition, re-orchestration, transposition of scenes, or conflation of the original and revised versions.

Several composers, chief among them Nikolai Rimsky-Korsakov and Dmitri Shostakovich, have created new editions of the opera to "correct" perceived technical weaknesses in the composer's original scores. Although these versions held the stage for decades, Mussorgsky's individual harmonic style and orchestration are now valued for their originality, and revisions by other hands have fallen out of fashion.

Boris Godunov comes closer to the status of a repertory piece than any other Russian opera, even Tchaikovsky's Eugene Onegin, and is the most recorded Russian opera.