Moshe Feinstein (Yiddish: משה פֿײַנשטײן; Lithuanian pronunciation: Moishe Fainshtein; English: Moses Feinstein; March 3, 1895 – March 23, 1986) was a Russian-born American Orthodox Jewish rabbi, scholar, and posek (authority on halakha—Jewish law). He has been called the most famous Orthodox Jewish legal authority of the 20th century and his rulings are often referenced in contemporary rabbinic literature. Feinstein served as president of the Union of Orthodox Rabbis, chairman of the Council of the Moetzes Gedolei HaTorah of the Agudath Israel of America, and head of Mesivtha Tifereth Jerusalem in New York.

Feinstein is commonly called "Reb Moshe" (or "Rav Moshe").

Moshe Feinstein was born, according to the Hebrew calendar, on Adar 7, 5655, in Uzda, Minsk Governorate, in the Russian Empire (now in Belarus). His father, David Feinstein, was the rabbi of Uzda and a great-grandson of the Vilna Gaon's brother. David Feinstein's father, Yechiel Michel Feinstein, was a Koidanover Chassid. His mother was a descendant of talmudist Yom-Tov Lipmann Heller, the Shlah HaKadosh, and Rashi. He studied with his father and in yeshivas in Slutsk, under Pesach Pruskin, and Shklov. He also had a close relationship with his uncle, Yaakov Kantrowitz, rabbi of Timkovichi, whom he greatly revered and considered his mentor. For the rest of his life, Feinstein considered Pruskin his rebbe.

Feinstein was appointed rabbi of Lyuban, where he served for 16 years. He married Shima Kustanovich in 1920 and had four children (Pesach Chaim, Fay Gittel, Shifra, and David) before leaving Europe. Pesach Chaim died in Europe, and another son, Reuven, was born in the United States. Under increasing pressure from the Soviet regime, in January 1937 he moved with his family to New York City, where he lived for the rest of his life.

Settling on the Lower East Side, Feinstein became the rosh yeshiva of Mesivtha Tifereth Jerusalem. He later established a branch of the yeshiva in Staten Island, New York, now headed by his son Reuven. His son Dovid headed the Manhattan branch.

Feinstein was president of the Union of Orthodox Rabbis of the United States and Canada and chaired the Moetzes Gedolei HaTorah of Agudath Israel of America from the 1960s until his death. Feinstein also took an active leadership role in Israel's Chinuch Atzmai.

Feinstein was recognized by many as the preeminent halakhic authority (posek) of his generation, ruling on issues of Jewish law as they pertain to modern times. People around the world called upon him to answer their most complicated halachic questions.

Owing to his prominence as an adjudicator of Jewish law, Feinstein was often asked to rule on very difficult questions, whereupon he often employed a number of innovative and controversial theories to reach his decisions. Soon after arriving in the United States, he established a reputation for handling business and labor disputes, writing about strikes, seniority, and fair competition. He later served as the chief halakhic authority for the Association of Orthodox Jewish Scientists, an indication of his expertise in Jewish medical ethics. In the medical arena, he opposed the early, unsuccessful heart transplants, although it has been reported off-the-record that in his later years, he allowed a person to receive a heart transplant after the medical technique of preventing rejection improved. On such matters, he often consulted with various scientific experts, including his son-in-law Moshe David Tendler, a professor of biology who served as a rosh yeshiva at Yeshiva University.