Mordecai Menahem Kaplan (Hebrew: מֹרְדֳּכַי מְנַחֵם קַפְלַן; June 11, 1881 – November 8, 1983) was an American Conservative rabbi, writer, Jewish educator, professor, theologian-philosopher, activist, and religious leader who founded the Reconstructionist movement of Judaism with his son-in-law, Rabbi Ira Eisenstein. He has been described as a "towering figure" in the recent history of Judaism for his influential work in adapting it to modern society, contending that Judaism should be a unifying and creative force by stressing the cultural and historical character of the religion as well as theological doctrine.

Mordecai Menahem Kaplan was born Mottel Kaplan in Sventiany in the Russian Empire (present-day Švenčionys in Lithuania) on June 11, 1881, the son of Haya (née Anna) and Rabbi Israel Kaplan. His father, who received semikhah (סְמִיכָה) by leading Lithuanian Jewish scholars, went to serve as a dayan (דַּיָּן, 'judge') in the court of Chief Rabbi Jacob Joseph in New York City in 1888. Mordecai was brought over to New York in 1889, at the age of nine.

Although affiliated with the most traditional Orthodox Jewish institutions and personalities on the Lower East Side, his father persisted in non-conformist openness to trends he had already exhibited in Russia: He hosted discussions in his home with maverick Hebrew Bible critic Arnold Ehrlich, withdrew his son from Etz Chaim Yeshiva in Manhattan, enrolled him in public school, and later sent him to the Jewish Theological Seminary of America (JTSA) to pursue modern Orthodox semikhah. Although not the norm amongst first-generation immigrants, who tended to be very conservative and traditional, his father was not alone in this kind of religious broad-mindedness. Kaplan's early education was strictly Orthodox, but, by the time he reached secondary school, he had been attracted to heterodox opinions (particularly regarding the critical approach to the Hebrew Bible). To counter this, Israel hired a tutor to study Maimonides's Guide for the Perplexed with Mordecai.

In 1893, Kaplan began studying for ordination at JTSA, which, at that time, was a Modern Orthodox Jewish institution founded to strengthen Orthodoxy and combat the hegemony of Reform Judaism. In 1895, he also began studies at the City College of New York (CCNY), which he attended in the morning; Kaplan attended JTSA in the evening. After graduating from CCNY in 1900, he went to Columbia University to study philosophy, sociology, and education, and received a master's degree and doctorate. Majoring in philosophy, he wrote his master's thesis on the ethical philosophy of Henry Sidgwick. His lecturers included the philosopher of ethical culture Felix Adler and the sociologist Franklin Henry Giddings.

In 1902, he was ordained at JTSA. Kaplan's understanding of Judaism differed from that of the seminary, but he maintained a long-standing connection with the institution, teaching there for more than fifty years. He became principal of its teachers' institute in 1909, served as dean in 1931, and retired afterwards 1963. In 1903, he was appointed as administrator of the religious school at Congregation Kehilath Jeshurun (KJ), a gradually modernizing Orthodox synagogue in Yorkville, Manhattan, consisting of newly affluent and acculturating East European Jews who had migrated north from the Lower East Side. By April 1904, he was appointed as rabbi of the congregation.

Based on his diary, by around this time (1904, age 23), Kaplan already had serious misgivings about Orthodoxy's ability to satisfy his spiritual needs and its unwillingness to modernize. By 1905, he had begun to doubt the divine inspiration of the Tanakh and its laws, as well as the efficacy of prayers and rituals; by 1907, he had informed his parents of his feelings. Because he was serving as a congregational rabbi, this created significant dissonance, resulting in considerable internal turmoil and anguish over the hypocrisy of practicing and preaching what he no longer believed. His private diaries and papers reveal that he was tortured within because his beliefs about the nature of religion and of Judaism conflicted with his duties as the leader of an Orthodox congregation.

In 1908, he married Lena Rubin, left KJ, and was ordained by Rabbi Yitzchak Yaacov Reines while on his honeymoon in Europe. In 1909, Kaplan became principal of the newly formed teachers' institute at JTSA (which had joined the Conservative movement), a position he held until his retirement in 1963. Kaplan was not primarily interested in academic scholarship, but rather in teaching future rabbis and educators to reinterpret Judaism and make Jewish identity meaningful under modern circumstances. As a result, his work during this time contributed greatly to the future of Jewish education in America.

Even those who disagreed with his views appreciated his direct approach. They were impressed by his emphasis on intellectual honesty in confronting the challenges posed by modern thought to traditional Jewish beliefs and practices. In his approach to midrash and philosophies of religion, Kaplan combined scientific scholarship with creative application of the texts to contemporary problems. Kaplan's Reconstructionist philosophy influenced not only his own immediate students but, through them, his extensive writings and public lectures over several decades, the American Jewish community at large. Many of his ideas, such as Judaism as a civilization (and not merely a religion or nationality); bat mitzvah; egalitarian involvement of women in synagogal and communal life; the synagogue as a Jewish center and not merely a place of worship; and living as Jews in a multicultural society, eventually came to be accepted as commonplace and implemented in all but strictly Orthodox segments of the community.