Eliezer ben Jacob I (Hebrew: אליעזר בן יעקב) was a Tanna of the 1st century; contemporary of Eleazar b. Ḥisma and Eliezer b. Hyrcanus, and senior of Judah ben Ilai (Pes. 32a, 39b; Yalḳ., Lev. 638). Of his personal history nothing is known, except that he had seen the Temple at Jerusalem and was familiar with the specific purposes of its many apartments, a subject on which he was considered an authority (Yoma 16b). Some of the details, however, he eventually forgot, and was reminded of them by Abba Saul b. Baṭnit (Mid. ii. 5, v. 4). Simon b. 'Azzai, R. Akiba's contemporary, relates that he had discovered a genealogical roll wherein was stated, "The Mishnah of R. Eliezer b. Jacob is only a 'ḳab' [small in proportion], but clear" (משנת ראב"י קב ונקי, Yeb. 49b), wherefore subsequent generations generally adopted Eliezer's views as law (Yeb. 60a; Bek. 23b).
In the Aggadah, too, he is mentioned. According to him, what the Bible says (Deut. xi. 13), "To serve him with all your heart and with all your soul," is an admonition to the priests that, when officiating, they shall entertain no thought foreign to their duty (Sifre, Deut. 41).