Iolanthe; or, The Peer and the Peri is a comic opera with music by Arthur Sullivan and libretto by W. S. Gilbert. It is one of the Savoy operas and is the seventh collaboration of the fourteen between Gilbert and Sullivan.

Iolanthe was first produced in London at the Savoy Theatre, on 25 November 1882, three days after Patience closed, and ran for 398 performances. It was the first of Gilbert and Sullivan's operas to premiere at the Savoy Theatre. The story concerns a band of immortal fairies who find themselves at odds with the House of Peers. The opera satirises many aspects of British government and law.