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.

Comic opera, sometimes known as light opera, is a sung dramatic work of a light or comic nature, usually with a happy ending and often including spoken dialogue.