Princess Ida, or Castle Adamant, is a comic opera with music by Arthur Sullivan and libretto by W. S. Gilbert. It opened at the Savoy Theatre on January 5, 1884, for a run of 246 performances. By Savoy Opera standards, it was not considered a success (a particularly hot summer in London did not help ticket sales), and it was not revived in London until 1919. This was the eighth collaboration between Gilbert and Sullivan.

Princess Ida is based on Tennyson's poem The Princess. Gilbert had written a blank verse parody of the same material in 1870. According to biographer Hesketh Pearson, Gilbert lifted parts of the dialogue from the earlier play into the libretto. Princess Ida thus became the only Gilbert and Sullivan work with dialogue entirely in blank verse. It is also the only one of their works in three acts.

The opera satirizes feminism, women's education, and Darwinian evolution, all of which were controversial topics in conservative Victorian England. Like the Patience and Iolanthe, Ida concerns the war between the sexes. Whereas, in Patience, the aesthetic-crazed women are contrasted with vain military men; and in Iolanthe, the vague and flighty fairies (women) are pitted against the ineffective, dim-witted peers (men); in Ida, overly serious students and professors at a women's university (women) defy a marriage-by-force ultimatum by a militaristic king and his testosterone-laden court (men).

Sullivan's score is majestic, and a sequence of songs in Act II, sometimes known as the "string of pearls", is particularly well loved. Although Gilbert's libretto contains some very funny lines, the iambic pentameter and three act structure tend to make it more difficult than some of the other Savoy Operas to keep the audience's interest to the end. In addition, modern audiences sometimes find the libretto's stereotyped portayal of sex roles and the awkward resolution of the opera unsatisfying.