Bún riêu is a Vietnamese noodle soup that combines freshwater crab paste with a tomato-based broth.
The crab mixture is traditionally made from small freshwater crabs, whose bodies are pounded or processed and strained to produce a crab-infused liquid. As it cooks, the proteins form savory curds that become a defining part of the soup.
Tomatoes provide acidity and color, while toppings can include rice vermicelli, tofu, herbs, vegetables, and sometimes pork or other seafood. Regional and restaurant versions vary considerably.
Bún riêu should not be confused with bún bò Huế, whose signature broth is beef-based and spicy, or phở, whose broth is built around beef or chicken and aromatic spices. The freshwater-crab element is the key identifying feature.