Otto Kotilainen (5.2. 1868 Heinävesi – 9.8. 1936 Helsinki) was a Finnish composer and teacher. He composed well-known Christmas carols such as Varpunen jouluaamuna and Kun joulu on, numerous choral songs and drama music.

Kotilainen initially worked in business, but in 1887 he went to study at the Jyväskylä Seminary, from which he graduated as a teacher in 1891. At the seminary he studied composition, choral conducting and singing under the teachers E. A. Hagfors and P. J. Hannikainen. In 1893 he was admitted to a teaching position in Helsinki, where he continued his composition studies at the Helsinki Music Institute (now the Sibelius Academy), the Helsinki Philharmonic Society Orchestra School and also as a student of Jean Sibelius.

Kotilainen later worked as a teacher at the Helsinki Music Institute and the Helsinki Conservatory, where he was a theory teacher from 1910. Kotilainen also wrote music reviews for, among others, Päivälehti and Helsingin Sanomat.