Koji is a traditional fermentation culture medium, based on Aspergillus oryzae, which is commonly used in the manufacture process of Japanese fermented products such as soy sauce, miso, and sake, and promote enzymatic degradation. Koji is usually prepared by culturing a mold on cereals such as wheat flour, soybean, or rice, but that cultured on seaweeds has not been developed yet. This study prepared the koji by culturing A oryzae on seaweed nori (dried piece of Pyropia yezoensis), and, then, characterized on this nori koji. The nori koji contained 0.85 lig N-acetylglucosamine, estimated as 6.1 sg mold cells, per gram dry matter and showed various kind of enzymatic activities in glycosidase, protease, and phosphatase as well as traditional soy sauce koji and rice koji. The suitability of these characteristics for degradation of nori was tested on nori sauce culture with and without the addition of the nori koji. After 167 days of culture, the fermentation tank with the nori koji showed over 74% recovery of supernatant while that without the nori koji had less than 57% recovery. The supernatant of culture mashes contained more than two times larger quantity of total nitrogen compounds in nori koji test group against control group. The present study prepared koji on seaweed nod for the first time and demonstrated its advantages to shorten the culture period and increase taste value in nori sauce manufacture. Development of seaweed koji enables a method to prepare cereal allergen free fermented sauces from seaweeds. (C) 2018, The Society for Biotechnology, Japan. All rights reserved.