No, a leek is not the same as a green onion. Although leeks and green onions do belong to the same genus, they are different species. Green onions are much smaller and thinner than leeks. Green onions and leeks both have a mild onion taste, however, leeks have a more subtle flavor and green onions have a sharper flavor. Typically, the white base and light green middle of leeks are eaten, and the green tops are discarded because they are harder to chew. With green onions, the entire plant is often eaten. However, the green tops are occasionally discarded as well. Despite their differences, there are many similarities between leeks and green onions, and they make great substitutes for each other.