Marie Catherine Sophie de Flavigny, Vicomtesse de Flavigny (December 31, 1805 - March 5, 1876), was a French author, known also by her married name and title, Marie, Comtesse d'Agoult, and by her pen name, Daniel Stern.

She was born in Frankfurt am Main, Germany, the daughter of Alexander Victor François de Flavigny (1770 - 1819), a footloose émigré French aristocrat, and his wife Maria Elisabeth Bethmann (1772 - 1847), a German banker's daughter. The young Marie spent her early years in Germany and completed her education in a French convent after the Bourbon Restoration. She entered into an early marriage of convenience with Charles Louis Constant d'Agoult, Comte d'Agoult (1790 - 1875) on May 16, 1827, thereby becoming the Comtesse d'Agoult. They had two daughters, Louise (1828 - 1834) and Claire (1830 - 1912). They were divorced on August 19, 1835.

From 1835 to 1839 she lived with virtuoso pianist and composer Franz Liszt, who was five years younger, and was then a rising concert star. D'Agoult had three children with Liszt; however, she and Liszt did not marry, maintaining their independent views and other differences while Liszt was busy composing and touring throughout Europe. Their children were Blandine (1835 - 1862), who was the first wife of future French prime minister Émile Ollivier but died at the age of 26; Cosima (1837 - 1930) (who married Richard Wagner, the second marriage for them both); and Daniel (1839 - 1859), who was already a promising pianist and gifted scholar when he died of tuberculosis. Chopin dedicated his second set of piano etudes to Marie d'Agoult.

In 1876 she died in Paris, and was buried in Division 54 of Père Lachaise Cemetery.

She was portrayed by Geneviève Page in the 1960 film Song Without End, opposite Dirk Bogarde as Liszt, and by Bernadette Peters in the 1991 James Lapine film Impromptu, which dramatized encounters between d'Agoult, Liszt (Julian Sands), Chopin (Hugh Grant), and Georges Sand (Judy Davis).

Her first stories (Hervé, Julien, Valentia, Nélida) were published in 1841 - 1845. Her best known work (written as "Daniel Stern") is the Histoire de la Révolution de 1848 (appearing from 1850 - 53, in 3 volumes). D'Agoult's other works include Lettres Républicaines in Esquisses morales et politiques (1849, collected articles), Trois journées de la vie de Marie Stuart (1856), Florence et Turin (1862), Histoire des commencements de la république aux Pays-Bas (1872), and Mes souvenirs (1877, posthumously).