Carl August Hirn (November 21, 1886, Säkkijärvi – February 16, 1949, Helsinki) was a Finnish composer and music editor.

Hirn’s parents were sawmill owner David Herman Hirn and Hilda Maria Olsoni. His uncle was Professor Yrjö Hirn. Hirn attended the 9th grade of Hamina comprehensive school and briefly studied at the Helsinki Music Institute. In 1910, he traveled to Vienna, recommended by Jean Sibelius, where he studied music theory, composition, and piano privately. From Vienna, Hirn moved to Berlin, where he studied at the Königliche Akademische Hochschule für Musik and privately.

The publishing house Simrock began printing Hirn’s compositions and also awarded him a Brahms scholarship. Hirn moved to Paris in 1927, where he studied privately in addition to composing. He also wrote about French musical life in German newspapers. In Paris, he lived in the artists’ district of Montparnasse, where salon orchestras also performed his compositions on the streets. In addition, his works were played on the radio in Berlin and Paris. Hirn lived abroad for a total of over 20 years. At first, he visited Finland at Christmas and in the summer, but before returning to Finland in 1933, he stayed abroad continuously for about ten years. Hirn had only intended to visit Finland, but the tense world situation and the war forced him to stay in the country.