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

She was born in Frankfurt-am-Main, Germany, the daughter of Alexander Victor François de Flavigny (1770-1819) and his wife Maria Elisabeth Bethmann (1772-1847).

She married 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, but did not marry, composer Franz Liszt (1811-1886). Their children were Blandina (1835-1862); Cosima (1837-1930) (who married Richard Wagner); and Daniel (1839-1859).

She died in Paris, and was buried in Division 54 of Père Lachaise Cemetery.

Her best-known work (written as "Daniel Stern") is the Histoire de la Révolution de 1848.