Cox and Box; or, The Long-Lost Brothers, is a one-act comic opera with a libretto by F. C. Burnand and music by Arthur Sullivan, based on the 1847 farce Box and Cox by John Maddison Morton. It was Sullivan's first successful comic opera. The story concerns a landlord who lets a room to two lodgers, one who works at night and one who works during the day. When one of them has the day off, they meet each other in the room and tempers flare. Sullivan wrote this piece five years before his first opera with W. S. Gilbert, Thespis.

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.