Leo Wiener was an American historian, linguist, author and translator.

Wiener was born in Białystok (then in the Russian Empire), of Polish-Jewish origin. His father was Zalmen (Solomon) Wiener, and his mother was Frejda Rabinowicz. He studied at the University of Warsaw in 1880, and then at the Friedrich Wilhelm University in Berlin. Wiener later declared, "Having 'for many years been a member of the Unitarian Church,' and having 'preached absolute amalgamation with the Gentile surroundings', [I] 'never allied with the Jewish Church or with Jews as such."

Wiener left Europe with the plan of founding a vegetarian commune in British Honduras (now Belize). He sailed steerage to New Orleans. On his arrival, in 1880, he had no money. After travel and work around the US, he went to Kansas City, Missouri, and became a lecturer in the department of Germanic and Romance languages at the University of Kansas. He was a polyglot, and was reputed to speak thirty languages well.

Wiener published articles on Yiddish linguistic elements in Polish, German, Ukrainian, and Belorussian. In 1898, Wiener traveled to Europe to collect material for his book The History of Yiddish Literature in the Nineteenth Century (1899). Isaac Peretz encouraged him and Abraham Harkavy, librarian at the Asiatic Museum of St. Petersburg, presented him with a thousand Yiddish books, which formed the basis of the Yiddish collection of the Harvard University library. After this project Wiener's interest in Yiddish declined.

Beginning in 1896, Wiener lectured on Slavic cultures at Harvard University and became the first American professor of Slavic literature. He compiled a valuable anthology of Russian literature and translated 24 volumes of Leo Tolstoy's works into English, a task which he completed in 24 months. He taught George Rapall Noyes.