Eliezer ben Nathan (Hebrew: אליעזר בן נתן) of Mainz (1090–1170), Ra'aven (ראב"ן), was a halakist and liturgical poet. As an early Rishon, he was a contemporary of the Rashbam and Rabbeinu Tam, and one of the earliest of the Tosafists. He was the son-in-law of Rabbi Eliakim b. Joseph of Mainz, a fellow student of Rashi. Through his four daughters Eliezer became the ancestor of several learned families which exerted a great influence upon religious life in the subsequent centuries. One of his great-grandsons was R. Asher b. Jehiel (ROSH), father of R. Jacob, author of the Ṭurim.
Eliezer proves himself conscientious and careful in his decisions. Unlike R. Tam, he possessed little self-confidence, and in his humility and reverence for tradition he is inclined to extremely rigid interpretations of the Law. Solomon's injunction (Prov. i. 8), "Forsake not the teaching of thy mother," he interprets as meaning, "What the older rabbis have prohibited we must not permit" (No. 10). The chapters on civil law contain many an interesting document, and also a statement of commercial relations occasioned by various trials.