Rabbi Menachem Mendel Yosef Zaks (Hebrew: מנדל זאקס; 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 Faiga Chaya, the youngest daughter of the Chofetz Chaim (who was 64 at the time of her birth). 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 Radun', 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. 
His sons Rabbis Gershon and Hillel Zaks (Rav in Kiryat Sefer and Rosh Yeshiva in Yeshivas Knesses Hagedolah) died in 1989 and 2015, respectively. His daughter, Rivka Wiesenfeld, died in 2008.
- Kobre, Eytan (Jun 2, 2014). "Living the Legend". Mishpacha (512): 44–57. Retrieved Jun 10, 2014.
- "The Yeshiva Chofetz Chaim Of Radin".
- http://www.yu.edu/riets/index.aspx?id=24114&ekmensel=51b41ad4_2186_2194_23718_2&AspxAutoDetectCookieSupport=1[dead link]
- הגאון רבי הלל זקס זצ"ל ראש ישיבת כנסת הגדולה. Kikar HaShabbat (in Hebrew). Jan 13, 2015. Retrieved Jan 14, 2015.
- "Levaya Of Rebbitzen Rivka Wiesenfeld A”H". Yeshiva World News. Jan 8, 2008. Retrieved Jun 10, 2014.
|This biographical article about an American rabbi is a stub. You can help Wikipedia by expanding it.|