Bluebeard's Castle (Hungarian: A kékszakállú herceg vára; literally: The Castle of the Blue-Bearded Duke) is a one-act opera by Hungarian composer Béla Bartók. The libretto was written by Béla Balázs, a poet and friend of the composer. It is in Hungarian, based on the French literary tale "La Barbe bleue" by Charles Perrault. The opera lasts only a little over an hour and there are only two singing characters onstage: Bluebeard (Kékszakállú), and his new wife Judith (Judit); the two have just eloped and Judith is coming home to Bluebeard's castle for the first time.

Bluebeard's Castle was composed in 1911 (with modifications made in 1912 and a new ending added in 1917) and first performed on 24 May 1918 in Budapest. Universal published the vocal (1921) and full score (1925). The Boosey & Hawkes' full score includes only the German and English singing translations while the Dover edition reproduces the Universal Edition Hungarian/German vocal score (with page numbers beginning at 1 instead of 5). A revision of the UE vocal score in 1963 added a new German translation by Wilhelm Ziegler, but seems not to have corrected any errata. Universal Edition and Bartók Records has published a new edition of the work in 2005 with new English translation by Peter Bartók, accompanied by extensive errata list.

Balázs originally conceived the libretto for his roommate Zoltán Kodály in 1908, and wrote it during the following two years. It was first published serially in 1910 with a joint dedication to Kodály and Bartók, and in 1912 appeared with the prologue in the collection "Mysteries". Bartók was motivated to complete the opera in 1911 by the closing date of the Ferenc Erkel Prize competition, for which it was duly entered. A second competition, organised by the music publishers Rózsavölgyi and with a closing date in 1912, encouraged Bartók to make some modifications to the work in order to submit it to the Rózsavölgyi competition.

Little is known about the Ferenc Erkel Prize other than that Bluebeard's Castle did not win. The Rózsavölgyi judges, after reviewing the composition, decided that the work (with only two characters and a single location) was not dramatic enough to be considered in the category for which it was entered: theatrical music. It is thought that the panel of judges who were to look at the musical (rather than the theatrical) aspects of the competition entries never saw Bartók's entry.

In 1913 Balázs produced a spoken performance at which Bartók played some piano pieces on a separate part of the program. A 1915 letter to Bartók's young wife, Márta, (to whom he dedicated the opera) ends: Now I know that I will never hear it in this life. You asked me to play it for you—I am afraid I would not be able to get through it. Still I'll try so that we may mourn it together.