Shemariah ben Mordecai was a German rabbi and tosafist of the first half of the 12th century, who was a pupil of the tosafist Isaac ben Asher. He was considered an especially eminent authority on religious rites (ba'al ma'asim), and seems to have written posḳim (decisions); no less a person than Jacob b. Meïr Tam consulted him on a difficult question (Or Zarua' on B. B. 199).