How do you say cereal in Japanese?
— 穀物

Generic synonyms for cereal in Japanese
grass草 (くさ, kusa)
Specialized synonyms for cereal in Japanese
barley大麦 オオムギ (oomugi)
cornトウモロコシ (tōmorokoshi)
maizeトウモロコシ (tōmorokoshi)
rice稲 (いね, ine)
ryeライムギ (raimugi)
wheat小麦 (こむぎ, komugi)