Menashe Klein (1924–2011) (Hebrew: ר' מנשה קליין), also known as the Ungvarer Rav (Yiddish: אונגווארער רב), was a Hasidic Rebbe and posek (arbiter of Jewish law). He authored 18 volumes of responsa, spanning over 50 years, entitled Mishneh Halachos. Additionally, he authored some 25 other seforim, including a commentary on Rabbi Simeon Kayyara's BeHag. Toward the end of his life, he relocated from Brooklyn, New York to Jerusalem.
Menashe Klein was born in 1924 in the town of Orlova (now known as Irlyava) near the city of Ungvar, Czechoslovakia (currently located in Ukraine). He studied in the yeshiva of the Rav of Ungvar, Rabbi Yosef Elimelech Kahane.
During World War II, he was incarcerated in Auschwitz-Birkenau, Auschwitz-Buna, and finally in Buchenwald. At Buchenwald, he was sent out to "Stein," a Nazi satellite camp at Eschershausen, but was listed in camp records as returned to Buchenwald, where he was liberated and where he completed a postwar military interview.
On June 2, 1945, he was evacuated by train with 427 other former Buchenwald inmates ages 7 to 17 – among them Yisrael Meir Lau, Naphtali Lau-Lavie, and Elie Wiesel – to France, where they boarded at a sanitarium in Écouis. He was transferred to Ambloy together with about 100 other boys who desired kosher facilities and a higher level of religious observance. This group was under the supervision of social worker Judith Hemmendinger, who attempted to re-acclimate the boys to normal living. The group was transferred to Taverny after Yom Kippur 1945. Klein immigrated to the United States in 1947.
After World War II, he served as Rav in the "Chevrah Liyadi" shul, (which was located in the Williamsburg section of Brooklyn) and Principal of Yeshivas Shearis Hapleitah, under the direction of Rabbi Yekusiel Yehudah Halberstam, Klausenberger Rebbe. In 1964, he founded Yeshiva Beis Shearim in Borough Park, Brooklyn, where he served as rosh yeshiva.
The Ungvarer Rov was active until old age. He had thousands of disciples.
In 2009, he stirred controversy. He published a responsa which, in part, denoted Chabad messianists as "apikoras" (heretics). At the time, he referred to them and was quoted; "This sect of crazies, which falsify the Torah and our sages' words, to say the Moshiach is dead but is really alive... these are things against our holy Torah."  Referring to the fringe movement, Chabad messianism, within Chabad that has adopted the late Rebbe, Menachem Mendel Schneerson as the Jewish Redeemer, or Jewish messiah.
- Feingold, Henry L. (1992). A time for healing: American Jewry since World War II. The Johns Hopkins University Press. p. 183. ISBN 978-0-8018-4347-1.
- See Hemmendinger and Krell, The Children of Buchenwald: Child Survivors of the Holocaust and Their Post-war Lives (Gefen, 2000).
- Based on Red Cross International Tracing Service documents at the USHMM.
- Schmidt, Shira, and Mantaka, Bracha. "A Prince in a Castle". Ami, September 21, 2014, pp. 136-143.
- See Kuntres Persumei Nissa, which can be found in the tenth volume of Mishne Halachos. (New York, 1987)
- This information can be found in the books of Mishneh Halachos that can be found on http://www.tshuvos.com/
- http://collive.com/show_news.rtx?id=3432 | May 6, 2009 Rabbi Klein Rips Messianics | retrieved May 18 2017
- Hemmendinger, Judith; Krell, Robert (2000). The Children of Buchenwald: Child Survivors of the Holocaust and Their Post-War Lives. Gefen Publishing House Ltd. ISBN 965229246X.