NISSENBAUM, ISAAC (1868–1942), rabbi, Hebrew writer, and religious Zionist in Poland. Born in Bobruisk, Belorussia, Nissenbaum was ordained as a rabbi. He settled in Minsk, where he began his Zionist activity. When the yeshivah of *Volozhin was closed in 1892, he became head of the secret nationalistic association of that yeshivah, Neẓaḥ Israel, an office which he held until 1894, when he moved to Bialystok. There he became Samuel *Mohilever's secretary. From then on he was a central figure in the Zionist movement, particularly among the Orthodox Jews. After Mohilever's death, Nissenbaum served as a Zionist preacher, traversing towns and townlets in Russia, Poland, Latvia, and Lithuania. He used midrashic elements in his Zionist preachings and had a considerable influence on Orthodox Jews. In 1900 he settled in Warsaw and became a regular preacher in synagogues and other places. He was an active member of Mizrachi from its beginning, a member of the executive of the Polish Zionist Organization, and one of the heads of the Jewish National Fund.
Beginning in 1889, Nissenbaum wrote many essays on current events, Zionism, and religious Zionism, as well as personal memories and several exegetical books. He was one of the editors of Ha-Ẓefirah, and after World War i, editor of Mizrachi's weekly in Poland. He edited a series of republished classical books in Jewish studies. The first explanatory pamphlet concerning the Jewish National Fund was written by him (1902). During World War ii he remained in the Warsaw ghetto and was murdered there.
Among his homilies are Derushim ve-Ḥomer li-Derush (1903), Derashot le-Khol Shabbatot ha-Shanah ve-ha-Mo'adim (1908, 19232), Hagut Lev (1911, 19252), and Imrei Derush (1926). In the field of religious Zionism he wrote Ha-Dat ve-ha-Teḥiyyah ha-Le'ummit (1920), Ha-Yahadut ha-Le'ummit (1920), and a monograph on Samuel Mohilever (1930). He also published an autobiography entitled Alei Ḥeldi (1929, 19692). In 1948 a selection of his writings was published in Israel under the editorship of E.M. Genichovsky, and in 1956 a selection of his letters was edited and published by I. Shapira.
I. Shapira, Ha-Rav Yiẓḥak Nissenbaum (1951).