H.M.S. Pinafore; or, The Lass That Loved a Sailor is a comic opera in two acts, with music by Arthur Sullivan and a libretto by W. S. Gilbert. It opened at the Opera Comique in London on 25 May 1878, and ran for 571 performances, which was the second-longest run of any musical theatre piece up to that time. H.M.S. Pinafore was Gilbert and Sullivan's fourth operatic collaboration and their first international sensation.

The story takes place aboard the Royal Navy ship HMS Pinafore. The captain's daughter, Josephine, is in love with a lower-class sailor, Ralph Rackstraw, although her father intends her to marry Sir Joseph Porter, the First Lord of the Admiralty. She abides by her father's wishes at first, but Sir Joseph's advocacy of the equality of humankind encourages Ralph and Josephine to overturn conventional social order. They declare their love for each other and eventually plan to elope. The Captain discovers this plan, but, as in many of the Gilbert and Sullivan operas, a surprise disclosure changes things dramatically near the end of the story.