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 of fourteen operatic collaborations by Gilbert and Sullivan. In the opera, the fairy Iolanthe has been banished from fairyland because she married a mortal; this is forbidden by fairy law. Her son, Strephon, is an Arcadian shepherd who wants to marry Phyllis, a Ward of Chancery. All the members of the House of Peers also want to marry Phyllis. When Phyllis sees Strephon hugging a young woman (not knowing that it is his mother – immortal fairies all appear young), she assumes the worst and sets off a climactic confrontation between the peers and the fairies. The opera satirises many aspects of British government, law and society. The confrontation between the fairies and the peers is a version of one of Gilbert's favourite themes: a tranquil civilisation of women is disrupted by a male-dominated world through the discovery of mortal love.

Iolanthe opened in London on 25 November 1882, at the Savoy Theatre to a warm reception, and ran for 398 performances, the fourth consecutive hit by Gilbert and Sullivan. It was the first work to premiere at the Savoy (although Patience had transferred to the theatre in 1881) and was the first new theatre production in the world to be illuminated entirely with electric lights, permitting some special effects that had not been possible in the era of gas lighting. The opera opened simultaneously in New York, and touring companies were sent around the UK and US to play the piece. The first Australasian touring production followed in 1885, and the opera was revived in London beginning in 1901. The D'Oyly Carte Opera Company toured the opera nearly continuously in repertory from 1891 until 1982, and made several recordings of the opera over that period. Numerous other professional and amateur productions have been given of this enduring work, and various other recordings have been issued.