Frederick Theodore Albert Delius CH (born Fritz Theodor Albert Delius; /ˈdiːliəs/; 29 January 1862 – 10 June 1934) was an English composer. Born in Bradford in the north of England to a prosperous mercantile family, he resisted attempts to recruit him to commerce. He was sent to Florida in the United States in 1884 to manage an orange plantation. He soon neglected his managerial duties, and in 1886 returned to Europe.

Having been influenced by African-American music during his short stay in Florida, he began composing. After a brief period of formal musical study in Germany beginning in 1886, he embarked on a full-time career as a composer in Paris and then in nearby Grez-sur-Loing, where he and his wife Jelka lived for the rest of their lives, except during the First World War.

Delius's first successes came in Germany, where Hans Haym and other conductors promoted his music from the late 1890s. In Delius's native Britain, his music did not make regular appearances in concert programmes until 1907, after Thomas Beecham took it up. Beecham conducted the full premiere of A Mass of Life in London in 1909 (he had premiered Part II in Germany in 1908); he staged the opera A Village Romeo and Juliet at Covent Garden in 1910; and he mounted a six-day Delius festival in London in 1929, as well as making gramophone recordings of many of the composer's works. After 1918, Delius began to suffer the effects of syphilis, contracted during his earlier years in Paris. He became paralysed and blind, but completed some late compositions between 1928 and 1932 with the aid of an amanuensis, Eric Fenby.

The lyricism in Delius's early compositions reflected the music he had heard in America and the influences of European composers such as Grieg and Wagner. As his skills matured, he developed a style uniquely his own, characterised by his individual orchestration and his uses of chromatic harmony. Delius's music has been only intermittently popular, and often subject to critical attacks. The Delius Society, formed in 1962 by his more dedicated followers, continues to promote knowledge of the composer's life and works, and sponsors the annual Delius Prize competition for young musicians.

Delius was born in Bradford in Yorkshire. He was baptised as Fritz Theodor Albert Delius, and used the forename Fritz until he was about 40. He was the second of four sons – there were also ten daughters – born to Julius Delius (1822–1901) and his wife Elise Pauline, née Krönig (1838–1929). Delius's parents were born in Bielefeld, Westphalia, and Julius's family, originally Dutch, had already lived for several generations in German lands near the Rhine. Julius's father, Ernst Friedrich Delius, had served under Blücher in the Napoleonic Wars. Julius moved to England to further his career as a wool merchant, and became a naturalised British subject in 1850. He married Elise in 1856.

The Delius household was musical; famous musicians such as Joseph Joachim and Carlo Alfredo Piatti were guests, and played for the family. Despite his German parentage, the young Fritz was drawn to the music of Chopin and Grieg rather than the Austro-German music of Mozart and Beethoven, a preference that endured all his life. The young Delius was first taught the violin by Rudolph Bauerkeller of the Hallé Orchestra, and had more advanced studies under George Haddock of Leeds.

Although Delius achieved enough skill as a violinist to set up as a violin teacher in later years, his chief musical joy was to improvise at the piano, and it was a piano piece, a waltz by Chopin, that gave him his first ecstatic encounter with music. From 1874 to 1878 he was educated at Bradford Grammar School, where the singer John Coates was his slightly older contemporary; Delius then attended the International College at Isleworth (just west of London) between 1878 and 1880. As a pupil he was neither especially quick nor diligent, but the college was conveniently close to the city for Delius to be able to attend concerts and opera.

Julius Delius assumed that his son would play a part in the family wool business, and for the next three years he tried hard to persuade him to do so. Delius's first job was as the firm's representative in Stroud in Gloucestershire, where he did moderately well. After being sent in a similar capacity to Chemnitz, he neglected his duties in favour of trips to the major musical centres of Germany, and musical studies with Hans Sitt. His father sent him to Sweden, where he again put his artistic interests ahead of commerce, coming under the influence of the Norwegian dramatists Henrik Ibsen and Gunnar Heiberg. Ibsen's denunciations of social conventions further alienated Delius from his commercial background. Delius was then sent to represent the firm in France, but he frequently absented himself from business for excursions to the French Riviera. After this, Julius Delius recognised that there was no prospect that his son would succeed in the family business, but he remained opposed to music as a profession, and instead sent him to America to manage an orange plantation.