Gustav Theodore Holst (21 September 1874 – 25 May 1934) was an English composer and was a music teacher for nearly 20 years. He is most famous for his orchestral suite The Planets. Having studied at the Royal College of Music in London, his early work was influenced by Ravel, Grieg, Richard Strauss, and fellow student Ralph Vaughan Williams, but most of his music is highly original, with influences from Hindu spiritualism and English folk tunes. Holst's music is well known for unconventional use of metre and haunting melodies.

Holst wrote almost 200 catalogued compositions, including orchestral suites, operas, ballets, concertos, choral hymns, and songs (see Selected works below).

Holst became music master at St Paul's Girls' School in 1905 and director of music at Morley College in 1907, continuing in both posts until retirement.

He was the brother of Hollywood actor Ernest Cossart and father of the composer and conductor Imogen Holst, who wrote a biography of him in 1938.