Leo The Lion Mathisen (10 October 1906 in Copenhagen, Denmark – 16 December 1969 in Copenhagen, Denmark) was a Danish jazz pianist, composer, arranger, singer and bandleader. He was a leading figure of the thriving Danish scene during the years around World War II—a period which has been labeled the Golden Age of Danish Jazz—and he is considered to be one of most significant and original jazz musicians of his day.

Mathisen’s main source of inspiration was Fats Waller, both for his piano playing and singing, but the later years of his career saw some Bebop influence. His nickname is a reference to his first name as well as MGM’s mascot Leo the Lion. He is also remembered for his characteristic appearance with his flat, centrally parted hair, his thin mustache and, most notably, his fondness for large cigars.

Father of radio host Ann-Britt Mathisen.