Welsh National Opera (WNO; Welsh: Opera Cenedlaethol Cymru) is an opera company based in Cardiff, Wales. WNO gave its first performances in 1946. The company began as a mainly amateur body and transformed into an all-professional ensemble by 1973. In its early days, the company gave a single week's annual season in Cardiff, gradually extending its schedule to become an all-year-round operation, with its own salaried chorus and orchestra. It has been described by The New York Times as "one of the finest operatic ensembles in Europe".

For most of its existence the company lacked a permanent base in Cardiff, but in 2004 it moved into the new Wales Millennium Centre, Cardiff Bay. The company tours nationally and internationally, giving more than 120 performances annually, with a repertoire of eight operas each year, to a combined audience of more than 150,000 people. Its most frequent venues other than Cardiff are Llandudno in Wales and Bristol, Birmingham, Liverpool, Milton Keynes, Oxford, Plymouth, and Southampton in England.

Singers who have been associated with the company include Geraint Evans, Thomas Allen, Anne Evans, and Bryn Terfel. Guest artists from other countries have included Joan Hammond, Tito Gobbi and Elisabeth Söderström. Among the conductors have been Sir Charles Mackerras, Reginald Goodall, James Levine and Pierre Boulez. The company has been led by Aidan Lang as its General Director since 2019.

Choral singing became increasingly popular in 19th-century Wales, principally owing to the rise of the eisteddfod as a symbol of its culture. The first Welsh National Opera Company was formed in 1890. A local newspaper commented that it was remarkable that "a race of people to whom vocal music is a ruling passion should not generations ago have established a permanent national opera". The company gave performances of operas by the Welsh composer Joseph Parry in Cardiff and on tour in Wales. The company, predominantly amateur with some professional guest singers from the London stage, gave numerous performances of Parry's Blodwen and Arienwen, composed in 1878 and 1890 respectively. An American tour was planned, but the company folded, and Parry's final opera, The Maid of Cefn Ydfa, was given at Cardiff by the Moody-Manners Opera Company in 1902.

A Cardiff Grand Opera Society ran from 1924 to 1934. It presented week-long annual seasons of popular operas including Faust, Carmen and Il trovatore, and like its predecessor was mainly an amateur body, with professional guest principals. Apart from the productions of these two enterprises, opera in Wales in the late 19th and early 20th centuries was generally presented by visiting companies from England.

In the 1930s Idloes Owen, a singing teacher and conductor, ran an amateur choir, the Lyrian Singers, based in Cardiff. In November 1941, together with John Morgan – a former Carl Rosa baritone – and Morgan's fiancée Helena Hughes Brown, Owen agreed to found the Lyrian Grand Opera Company, with Brown as secretary and Owen as conductor and general manager. They publicised their plan and held a general meeting of potential supporters in December 1943; at that meeting the name of the proposed organisation was changed to "Welsh National Opera Company". By January 1944 plans were far enough advanced for the company's first rehearsals to be held. Owen recruited a local businessman, W. H. (Bill) Smith (1894–1968), who agreed to serve as business manager. At first doubtful of the company's prospects, Smith became its dominant influence, leading fund-raiser, and chairman for twenty years from 1948.

The new company made its debut at the Prince of Wales Theatre, Cardiff on 15 April 1946 with a double bill of Cavalleria rusticana and Pagliacci. The orchestra was professional, mostly drawn from members of the BBC Welsh Orchestra; all the singers were amateurs, except for Tudor Davies, a tenor well known at Covent Garden and Sadler's Wells, who sang Canio in Pagliacci. During the week-long season the new company also staged Faust, with Davies in the title role. Although nearing the end of his career he was a considerable box-office draw, and the company played to full houses. Nevertheless, the expense of a professional orchestra and the hire of costumes and scenery outweighed the box-office receipts, and the season made a small loss. Finance remained a recurring problem over the succeeding decades.

The sets were faded stock with years of amateur service behind them, there were one or two experienced guest principals … and a majority of local amateurs, stiff actors, sometimes vocally overparted, absurdly costumed, yet almost all of them ejecting the primeval essence of operatic enjoyment, a passion for and in singing.