Koji is commonly used in manufacturing Japanese fermented products and promotes enzymatic degradation. In recent years, a seaweed koji has been developed by culturing Aspergillus oryzae on nori Pyropia yezoensis. In the present study, we prepared nori koji by culturing A. oryzae on high- and low-quality noris and then mixing this preparation with additional dried nori. Suitable fermentation conditions for increasing the taste-active components of the nori mixed with nori koji were investigated. The mixture of nori and koji prepared from high-quality nori, with added water, cultured for 120 days at 10 °C, provided the greatest increase (7 times) in free amino acids. The changes in taste after culturing the nori and koji mixture were evaluated using a taste-sensing system. The sourness score of the nori and koji mixture increased significantly, but the scores for other attributes such as bitterness and umami richness did not increase after culture. The present study has demonstrated a clear increase in the free amino acid content of nori and a modification in the taste score by aging the culture with nori koji. These results will encourage the development of ‘aged seaweed,’ a novel value-added product with nutritional and taste elements modified using seaweed koji.