Mary Frances "Fanny" Ronalds (August 23, 1839 - July 28, 1916) was an American socialite and amateur singer who is best known for her long affair with the composer Arthur Sullivan in London in the last decades of the nineteenth century.
After separating from her husband, Ronalds moved to Paris in 1867 and then to London in 1871, being accepted into royal social circles and becoming a popular hostess in both cities. A noted beauty, she became romantically involved with Arthur Sullivan during the 1870s and continued as his companion until his death in 1900. She was much admired as a singer and became famously associated with one of Sullivan's most popular songs, "The Lost Chord".