The main ingredients of Miso soup are naturally vegan. Those are miso itself, which is soy-based, and koji, which is essentially fermented rice or fermented soy beans. Koji is a key ingredient for many famous Japanese products: sake (alcoholic drink), soy sauces, amazake beverage, rice vinegar and mirin seasoning. Koji gives dishes a profound umami taste: a pleasant, savory flavor. It is one of the five basic tastes along with sweet, bitter, salty, and sour. The history of Miso soup began sometime in the 12th century. It emerged as a type of fast food for the military at the time. It also became a popular soup-party main dish.
Miso soup is very filling and ideal for a calorie-conscious, high-protein diet. Japanese cuisine is considered the “cleanest” among the world diets which is one of the reasons why Japanese people have a longer life expectancy.
Miso itself is a fermented soybean paste. It dates back to sometime in 700 AD, when it became a sort of luxury seasoning for the noble and rich. It was consumed by itself, as well as spread on food. Miso as a seasoning was replaced eventually by soy sauce.
Though it sounds vegan, miso soup served in restaurants and other food outlets is made with dashi broth. Dashi is a fish stock usually made from kelp and bonito flakes or tuna.
If you want to have miso soup that is guaranteed vegan or vegetarian, it is better to make your own. The good news is that it is really easy to cook and will take you less than three minutes.
A vegan Miso soup can be water-based or made with dried kelp broth, called Kombu Dashi. Both options taste great, however Kombu Dashi adds a certain ocean accent to the flavor of Miso soup.

Miso soup can be of various colors depending on the type of grain used alongside soybeans and the duration of fermentation. The rule of thumb is: the darker the color, the stronger the taste. White Miso (Shiro) or mixed Miso (Awase) are the most popular soups. Darker versions have an intense taste and go better with some (vegan) sour cream or yogurt, mixed into the soup.
Also Miso soup goes well with tofu and wakame

Enjoy your healthy Japanese meal!