Mordecai Kaplan, Lithuanian rabbi, founded Reconstructionist Judaism (died 1983)

Mordecai Menahem Kaplan was an American Conservative rabbi, writer, Jewish educator, professor, theologian, philosopher, activist, and religious leader who founded the Reconstructionist movement of Judaism along with his son-in-law 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.

Reconstructionist Judaism is a Jewish movement based on the concepts developed by Rabbi Mordecai Kaplan (1881–1983)—namely, that Judaism is a progressively evolving civilization rather than just a religion. The movement originated as a semi-organized stream within Conservative Judaism, developed between the late 1920s and the 1940s before seceding in 1955, and established a rabbinical college in 1967. Reconstructionist Judaism is recognized by many scholars as one of the five major streams of Judaism in America alongside Orthodox, Conservative, Reform, and Humanistic.