Leo "The Lion" Mathisen (10 October 1906 – 16 December 1969) 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 labelled the Golden Age of Danish Jazz—and he is considered to be one of the most significant and original jazz musicians of his day.

Mathisen's main source of inspiration was Fats Waller, both his piano play and his singing, but the later years of his career saw some bebop influence. His most famous tracks are "Take It Easy" and "To Be or Not to Be". 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 moustache and, most notably, his fondness of large cigars.