The primary ingredient used in traditional Japanese miso soup is miso paste, which is made from fermented soybeans. Miso is a critical component in Japanese cuisine, adding depth and umami flavor to dishes. The soup typically consists of dashi (a broth made from seaweed and fish) combined with miso paste, creating a savory base. Additional ingredients may include tofu, seaweed, and various vegetables like green onions or mushrooms, each contributing to the soup's texture and taste. Miso soup is often served as a staple in Japanese meals due to its nutritional benefits and comforting qualities.