Ruben Mattias Liljefors (30 September 1871, Uppsala – 4 March 1936, Uppsala) was a Swedish composer and conductor, brother of the artist Bruno Liljefors.
Liljefors studied in Uppsala with Ivar Eggert Hedenblad until 1895, and subsequently in Leipzig with Salomon Jadassohn until 1899. Later he attended the Stockholm Conservatory. He completed his education with Felix Draeseke, Max Reger, and Hermann Ludwig Kutzschbach. From 1902 to 1911, he conducted the choir and orchestra of the Gothenburg Philharmonic Society. Afterwards, he conducted the Gävleborg County Orchestra between 1912 and 1931.
He composed a symphony, a concert overture, a festival overture, an orchestral suite, a piano concerto, and a violin sonata. One of his best-known pieces is the Swedish Christmas carol När det lider mot jul. His son, Ingemar Liljefors, was also a successful composer, and his grandson, Mats Liljefors, is a well known conductor and violinist.