Letter from an Unknown Woman (German: Brief einer Unbekannten) is a novella by Stefan Zweig. Published in 1922, it tells the story of an author who, while reading a letter written by a woman he does not remember, gets glimpses into her life story.

A rich and well-known writer (R.), returning home to Vienna from one of many holidays, finds a long letter from an unknown woman (Fräulein). As a teenager the woman had lived with her poor widowed mother in the same building and had fallen totally in love with both the opulent cultured lifestyle of her neighbour and the handsome charming man himself. Her passion for the writer was not lessened by the flow of attractive women spending the night with him nor when she had to leave Vienna and moved to Innsbruck when her mother remarried. At age 18 she returned to Vienna, took a job and tried to meet the writer again. He did not recognise her and, without revealing her name, she succeeded in spending three nights with him before he disappeared on a long holiday. Pregnant, she lost her job and had to give birth in a refuge for the indigent. Resolved that their child should have a good life, she spent nights with or became mistress of various rich men but would never marry because her heart belonged always to the writer. Out one night with a current lover, she saw the writer in a night club and went home with him instead. To him, she was just an agreeable companion for that night, as he again did not recognise her. In the 1918 flu pandemic, the child died and she, ill herself, wrote the letter to be posted after her death.

In 1948, a film version was produced with a screenplay adaptation by Howard Koch. Starring Joan Fontaine, Louis Jourdan, Mady Christians and Marcel Journet, it was directed by Max Ophüls. In 1992, Letter from an Unknown Woman was selected for preservation in the United States National Film Registry by the Library of Congress as being "culturally, historically, or aesthetically significant".