German pianist and composer, the eldest sister and confidante of the composer Felix Mendelssohn (Mendelssohn, Felix).

Fanny is said to have been as talented musically as her brother, and the two children were given the same music teachers. Felix readily admitted that his sister played the piano better than he did, and Fanny remained his chief musical adviser until he left home. She is said to have memorized J.S. Bach's complete Well-Tempered Clavier by age 13.

Fanny married the Prussian court painter Wilhelm Hensel in 1829. She traveled in Italy with her husband in 1839–40. Upon her mother's death in 1842 she took over the direction of the Mendelssohn family home in Berlin, in which role she organized local concerts and occasionally appeared as a pianist. Fanny remained very close to her brother, and her death in May 1847 greatly contributed to Felix's own demise six months later.

Fanny wrote about 500 musical compositions in all, including about 120 pieces for piano, many lieder (art songs), and chamber music, cantatas, and oratorios. Six of her songs were published under Felix's name in his two sets of Twelve Songs (Opuses 8 and 9), while the few works published under her own name include several collections of short piano pieces, some lieder, and a piano trio. Most of her remaining works exist only in manuscript. Stylistically her music is similar to that of her brother.

Fanny's son Sebastian wrote a biography of the Mendelssohn family based partly on Fanny's diaries and letters, which provide a great deal of information about her brother Felix.