Gustaf Allan Pettersson (19 September 1911 – 20 June 1980) was a Swedish composer and violist. He is considered one of the 20th century's most important Swedish composers and was described as one of the last great symphonists, often compared to Gustav Mahler. Pettersson's music has a very distinctive sound and can hardly be confused with that of any other 20th-century composer. In the final decade of his life, his symphonies (typically one-movement works) developed an international following, particularly in Germany and Sweden. Of these, his best known work is Symphony No. 7. His music later found success in the United States. Gustaf Allan Pettersson was the youngest of four children. His father, Karl Viktor Pettersson (1875–1952), was a violent, alcoholic blacksmith, in Stockholm's Södermalm district, where he lived throughout his life. He once said:
With his parents and siblings, Pettersson lived in a damp, one-room basement apartment with bars on the window. Through strict self-discipline and with the help of music, Pettersson freed himself from his social misery and difficult family circumstances. He later made two unsuccessful attempts to enter the conservatory of the Royal Swedish Academy of Music.
In 1930, Pettersson began studying violin and later the viola, as well as counterpoint and harmony, at the conservatory of the Royal Swedish Academy of Music (Royal College of Music, Stockholm).
In September 1951, Pettersson went to Paris to study composition and was a student of composers René Leibowitz, Arthur Honegger, Olivier Messiaen, and Darius Milhaud. He returned to Sweden at the end of 1952. In the early 1950s, he was diagnosed with rheumatoid arthritis.