Fanny Mendelssohn (14 November 1805 – 14 May 1847) was a German composer and pianist of the early Romantic era who was known as Fanny Hensel after her marriage. Her compositions include a string quartet, a piano trio, a piano quartet, an orchestral overture, four cantatas, more than 125 pieces for the piano and over 250 lieder, most of which were unpublished in her lifetime. Although lauded for her piano technique, she rarely gave public performances outside her family circle.

She grew up in Berlin and received a thorough musical education from teachers including her mother, as well as the composers Ludwig Berger and Carl Friedrich Zelter. Her younger brother Felix Mendelssohn, also a composer and pianist, shared the same education and the two developed a close relationship. Owing to her family's reservations and to social conventions of the time about the roles of women, six of her songs were published under her brother's name in his Opus 8 and 9 collections. In 1829, she married artist Wilhelm Hensel and, in 1830, they had their only child, Sebastian Hensel. In 1846, despite the continuing ambivalence of her family towards her musical ambitions, Fanny Hensel published a collection of songs as her Opus 1. She died of a stroke in 1847, aged 41.

Since the 1990s, her life and works have been the subject of more detailed research. Her Easter Sonata was inaccurately credited to her brother in 1970, before new analysis of documents in 2010 corrected the attribution. The Fanny & Felix Mendelssohn Museum opened on 29 May 2018 in Hamburg, Germany.