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.

Sir William Schwenck Gilbert was an English dramatist, librettist, poet and illustrator best known for his collaboration with composer Arthur Sullivan, which produced fourteen comic operas. The most famous of these include H.M.S. Pinafore, The Pirates of Penzance and one of the most frequently performed works in the history of musical theatre, The Mikado. The popularity of these works was supported for over a century by year-round performances of them, in Britain and abroad, by the repertory company that Gilbert, Sullivan and their producer Richard D'Oyly Carte founded, the D'Oyly Carte Opera Company. These Savoy operas are still frequently performed in the English-speaking world and beyond.