Gustav Theodore Holst (born Gustavus Theodore von Holst; 21 September 1874 – 25 May 1934) was an English composer, arranger and teacher. Best known for his orchestral suite The Planets, he composed many other works across a range of genres, although none achieved comparable success. His distinctive compositional style was the product of many influences, Richard Wagner and Richard Strauss being most crucial early in his development. The subsequent inspiration of the English folksong revival of the early 20th century, and the example of such rising modern composers as Maurice Ravel, led Holst to develop and refine an individual style.

There were professional musicians in the previous three generations of Holst's family and it was clear from his early years that he would follow the same calling. He hoped to become a pianist, but was prevented by neuritis in his right arm. Despite his father's reservations, he pursued a career as a composer, studying at the Royal College of Music under Charles Villiers Stanford. Unable to support himself by his compositions, he played the trombone professionally and later became a teacher—a great one, according to his colleague Ralph Vaughan Williams. Among other teaching activities he built up a strong tradition of performance at Morley College, where he served as musical director from 1907 until 1924, and pioneered music education for women at St Paul's Girls' School, where he taught from 1905 until his death in 1934. He was the founder of a series of Whitsun music festivals, which ran from 1916 for the remainder of his life.

Holst's works were played frequently in the early years of the 20th century, but it was not until the international success of The Planets in the years immediately after the First World War that he became a well-known figure. A shy man, he did not welcome this fame, and preferred to be left in peace to compose and teach. In his later years his uncompromising, personal style of composition struck many music lovers as too austere, and his brief popularity declined. Nevertheless, he was an important influence on a number of younger English composers, including Edmund Rubbra, Michael Tippett and Benjamin Britten. Apart from The Planets and a handful of other works, his music was generally neglected until the 1980s, when recordings of much of his output became available.