Carl August Nielsen (Danish: [ˈkʰɑˀl ˈne̝lsn̩]; 9 June 1865 – 3 October 1931) was a Danish composer, conductor and violinist, widely recognized as his country's most prominent composer.

Brought up by poor yet musically talented parents on the island of Funen, he demonstrated his musical abilities at an early age. He initially played in a military band before attending the Royal Danish Academy of Music in Copenhagen from 1884 until December 1886. He premiered his Op. 1, Suite for Strings, in 1888, at the age of 23. The following year, Nielsen began a 16-year stint as a second violinist in the Royal Danish Orchestra under the conductor Johan Svendsen, during which he played in Giuseppe Verdi's Falstaff and Otello at their Danish premieres. In 1916, he took a post teaching at the Royal Danish Academy and continued to work there until his death.

Although his symphonies, concertos and choral music are now internationally acclaimed, Nielsen's career and personal life were marked by many difficulties, often reflected in his music. The works he composed between 1897 and 1904 are sometimes ascribed to his "psychological" period, resulting mainly from a turbulent marriage with the sculptor Anne Marie Brodersen. Nielsen is especially noted for his six symphonies, his Wind Quintet and his concertos for violin, flute and clarinet. In Denmark, his opera Maskarade and many of his songs have become an integral part of the national heritage. His early music was inspired by composers such as Brahms and Grieg, but he soon developed his own style, first experimenting with progressive tonality and later diverging even more radically from the standards of composition still common at the time. Nielsen's sixth and final symphony, Sinfonia semplice, was written in 1924–25. He died from a heart attack six years later, and is buried in Vestre Cemetery, Copenhagen.

Nielsen maintained the reputation of a musical outsider during his lifetime, both in his own country and internationally. It was only later that his works firmly entered the international repertoire, accelerating in popularity from the 1960s through Leonard Bernstein and others. In Denmark, Nielsen's reputation was sealed in 2006 when four of his works were listed by the Danish Ministry of Culture amongst the greatest pieces of Danish classical music. For many years, he appeared on the Danish hundred-kroner banknote. The Carl Nielsen Museum in Odense documents his life and that of his wife. Between 1994 and 2009 the Royal Danish Library, sponsored by the Danish government, completed the Carl Nielsen Edition, freely available online, containing background information and sheet music for all of Nielsen's works, many of which had not been previously published.