Menachem Mendel Schneersohn also known as the Tzemach Tzedek was an Orthodox rabbi, leading 19th-century posek, and the third rebbe of the Chabad Lubavitch Hasidic movement.

In Jewish law, a posek is a legal scholar who determines the application of halakha, the Jewish religious laws derived from the written and Oral Torah in cases of Jewish law where previous authorities are inconclusive, or in those situations where no clear halakhic precedent exists.