Rabbi Menachem Mendel Yosef Zaks (1898–1974) (commonly known as Rabbi Mendel Zaks) is best known for being the Rosh Yeshiva in Raduń Yeshiva and being the son-in-law to Rabbi Yisrael Meir Kagan, the Chofetz Chaim.
Rabbi Zaks was born in Siluva. Before joining the Yeshiva in Radun, Rabbi Zaks studied in the Yeshiva Beis Yitzchok (while it was still in Slabodka, before it moved to Kaminetz). He was forced to leave Radun during World War I and then studied under Rabbi Reuven Dov Dessler, the father of Rabbi Eliyahu Eliezer Dessler and brother-in-law to Rabbi Chaim Ozer Grodzinski. He also studied under Rabbi Meir Simcha of Dvinsk whose Meshech Chochmah he had helped print.
After the war, he returned to Radun where he studied with his future father-in-law. In 1922, he married the daughter of the Chofetz Chaim. By doing so, he joined the Chofetz Chaim as leader of the yeshiva and in all other communal affairs. Rabbi Zaks was immediately recognized for his teaching and leadership abilities. His vast knowledge of Talmudic Law, coupled with a photographic memory and unending hours spent with his students attracted even more students to the Radin Yeshiva at that time .
In 1925, Rabbi Zaks incorporated the Yeshiva in the United States in an effort to raise money there for the operation of the Yeshiva and its charities.
Upon the death of Naftoli Trop in 1928, Rabbi Mendel Zaks was appointed Rosh Yeshiva in the Yeshiva in Raduń, Poland. The outbreak of World War II forced the Yeshiva to transfer to Vilna, and eventually Rabbi Zaks re-established the Yeshiva in 1943 at New York City. In 1941 he moved to New York. In 1946, he accepted the position of rosh yeshiva and bochen (examiner) at Rabbi Isaac Elchanan Theological Seminary. Following his retirement, he died in Jerusalem in 1974.
Rabbi Zaks had an extraordinary memory, a pure mind, and was an expert in the Talmud and its commentaries. 
He is survived by his son Rabbi Hillel Zaks, Rav in Kiryat Sefer and Rosh Yeshiva in Yeshivas Knesses Hagdolah. His son, Rabbi Gershon Zaks, died in 1989 and his daughter, Rivka Wiesenfeld, died in 2008 
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