Fanny Cacilie Mendelssohn-Bartholdy (November 14, 1805–May 14, 1847), later Fanny Hensel, was a German pianist and amateur composer. She is perhaps best known as the sister of Felix Mendelssohn, but her own achievements are being increasingly recognised as significant in themselves.

Born in Hamburg, Fanny benefited from the same musical education and upbringing as her better known brother. Like him, Fanny showed prodigious musical ability as a child and began to write music. However, she was limited by prevailing attitudes of the time against women, attitudes apparently shared by her father and brother, who were tolerant, rather than supportive, of her activities as composer.

In 1829, after a courtship of several years, she married the painter Wilhelm Hensel who was a good deal more supportive of her composing. Subsequently, her works were often played alongside her brother's at the family home in Berlin in the very popular concerts which were held there.

As a pianist, Fanny became a supporter of her brother's compositions. Her public debut at the piano came in 1838, when she played Felix's piano concerto No. 1.

Her compositions include a piano trio and several books of solo piano pieces and songs. A number of her songs were originally published under Felix's name in his opus 8 and 9 collections. Her piano works are often in the manner of songs, and carry the name Lieder ohne Worte (Songs without Words). This style of piano work is commonly thought to have been developed by Felix Mendelssohn, though many scholars nowadays believe it was actually Fanny who first worked in the genre.

Fanny Mendelssohn-Hensel died in Berlin in 1847 of complications of a stroke suffered while rehearsing one of her brother's oratorios. In recent years, her music has become a little more prominent thanks to a number of CDs being released on labels such as Hyperion and CPO.