Lou Koster (7 May 1889 – 17 November 1973) was a Luxembourgish composer and pianist. Specializing in violin and piano, she studied at the Luxembourg Conservatory until 1906, then taught there from 1908. While still young, she began to compose. Her first success was her one-act operetta An der Schemm (At the Baths), composed in 1922. From 1933, many of her orchestral works, especially her waltzes and marches, were played for radio by the RTL Grand Symphony Orchestra. Her greatest success was however the choral ballad Der Geiger von Echternach (The Echternach Fiddler) which she completed when she was 85. It was premiered in the Abbey of Echternach in July 1972 to great acclaim.