Lodewijk Mortelmans (February 5, 1868 - June, 24 1952) was a Belgian composer and conductor of Flemish ancestry. Sometimes called de Vlaamse Brahms ("the Flemish Brahms"), Mortelmans composed in a number of forms, including piano music and orchestral works, but he was most celebrated in his day for his art songs.

Mortelmans studied music at the Royal Flemish Conservatory in Antwerp, where his teachers included Peter Benoit, Joseph Tilborghs, and Jan Blockx, as well as Arthur De Greef (piano) and Hubert Ferdinand Kufferath (counterpoint). In 1893, he was a winner of the Belgian Prix de Rome with his cantata Lady Macbeth. Both Mortelmans and his brother were members of the arts group De Scalden. Mortelmans was also affiliated with the arts groups Studie and De Kapel, and the arts journal Van Nu en Straks.

From 1901, Mortelmans taught counterpoint and fugue at the Royal Flemish Conservatory, and became its director on 6 September 1924. He retired from the post in 1933. His students included Lodewijk De Vocht, Marinus De Jong and Flor Peeters.