מרדכי בן ניסן הזקן, Mordechai ben Nissan ha-Zaken) was a Karaite Jewish scholar who lived at Krasny Ostrów, Poland (now Kukeziv, Ukraine) in the second half of the 18th century.
Mordecai is chiefly known through his work "Dod Mordechai" (Vienna, 1830), written in answer to four questions addressed in 1698 to David ben Shalom ha-Zaken by Jacobus Trigland, professor of theology at the University of Leiden.
These four questions were: (1) Is the Karaite sect identical with that which existed at the time of the Second Temple under the name "Sadducees," or did it originate with Anan ben David, as the Rabbinites assert?
(2) Was Aquila, the proselyte, to whom Menahem Qala'i had addressed letters, identical with the Greek translator or with the author of the Targum?
The Karaite Bible, he says, in answer to the fourth question, does not vary from that of the Rabbinites; and the vowels and accents are believed to have been transmitted to Moses on Mount Sinai.