Gaon of Sura from 801 to 815; succeeded Hilai ben Mari. He officiated fourteen years, according to a text of Sherira ("M. J. C." 1:39); according to other authorities (c. 1:65,188), eighteen years. In his decisions Jacob ben Mordecai leaned as much as possible toward the milder interpretation of the Law, for which Zadok (appointed gaon in 823) and his contemporaries blamed him ("Ḥemdah Genuzah," ed. Jerusalem, No. 8; "Sefer ha-Eshkol," 1:91). A long responsum of his is preserved in "Or Zarua'" (, No. 411; comp. also Rosh to Ḥul. , No. 14). His decisions are given in comparatively pure Hebrew.