The Norwegian opera singer Kirsten Malfrid Flagstad (July 12, 1895 – December 7, 1962) was considered one of the greatest Wagnerian sopranos of the 20th century. While not a magnetic stage performer, she was admired for her voice's sheer tonal beauty and consistency of line and tone.

Flagstad born at Hamar in a musical family; her father was conductor Michael Flagstad and her mother pianist Marie Flagstad Johnsrud. She received her early musical training in Oslo and made her stage debut as Nuri in Eugen d'Albert's Tiefland in 1913.

After further study in Stockholm, she began a career in operetta in Norway. In 1921, she made an unspectacular tour of France and was engaged in the city theater of Göteborg, Sweden from 1928 to 1932.

After singing operetta for over a decade, Flagstad was convinced to take on heavier operatic roles, including eventually Wagner. In 1932, she took on the role of Isolde and appeared to have found her true voice. Ella Gulbranson, a Norwegian singer, convinced Winifred Wagner to audition Flagstad for Bayreuth. Flagstad was hired for minor roles the first year, and then for the role of Sieglinde in Die Walküre.

Her Metropolitan Opera début as Sieglinde, broadcast nationwide on February 2, 1935, created a sensation. Four days later, Flagstad sang Isolde, and later that month, she performed Brünhilde in Die Walküre and Götterdämmerung for the first time. Later that season, Flagstad sang Elsa, Elisabeth, and her first Kundry. Almost overnight, she had established herself as the pre-eminent Wagnerian soprano of the era. Fidelio (1936) was her only non-Wagnerian role at the Met before the war. In 1936, she performed all three Brünnhildes in the San Francisco Opera's Ring cycle. In 1937, she appeared at the Lyric Opera of Chicago.

In 1936 and 1937, Flagstad performed the roles of Isolde, Brünhilde and Senta at the Royal Opera House, Covent Garden under Sir Thomas Beecham, Fritz Reiner and Wilhelm Furtwängler, arousing as much enthusiasm there as in New York.

In 1940, with the outbreak of World War II, Flagstad returned to Norway to be with her husband and remained there throughout the war. Although she never sang for the German occupiers, her husband was arrested for war profiteering after the war because he had sold supplies to the Germans. He was already seriously ill and died shortly thereafter, whereupon his estate was impounded by the state. Although her own wartime record was free from controversy, Flagstad's return to Norway during the war and a certain political naivety on her part created ill-feeling towards her, particularly in the United States.

During four consecutive Covent Garden seasons, from 1948 to 1951, Flagstad repeated all her regular Wagnerian roles, including Kundry and Sieglinde. She toured South America and also returned to San Francisco in 1948 but was not invited back to the Met until Sir Rudolph Bing became manager. In the 1950-1951 season, although she was well into her 50s, Flagstad showed herself still in remarkable form as Isolde, Brünnhilde and Fidelio.

She gave her farewell performance at the Met on April 1, 1952 as the title role in Glück's Alceste. Her final operatic performances was as Purcell’s Dido at the Mermaid Theatre in London in 1953, a performance that was recorded and issued by EMI.

After her retirement from the stage, she continued to give concert performances and record. From 1958 to 1960, Flagstad was the general manager of the Norwegian National Opera

Of her many recording, the complete Tristan und Isolde with Furtwängler is considered the finest representation of her interpretive art in its maturity. Her pre-war recordings, however, are said to showcase her voice in its freshest brilliance and clarity.

Flagstad died in Olso at the age of 67. Her portrait appears on the Norwegian 100 kroner bill.

A Kirsten Flagstad Museum in her birthplace contains a private collection of opera artifacts thought to be the largest in the world. Her costumes draw special attention, since in her day, each singer travelled with "her" Brünnhilde costume, etc.