Bún mắm is the Vietnamese noodle soup from the Mekong Delta that uses a fermented fish broth and is commonly topped with pork, shrimp and fish.
Its broth is built from mắm cá, a fermented fish paste or sauce characteristic of the Mekong Delta. The fermented base gives the soup a deep, savory aroma and flavor. Cooks usually dilute and strain it before adding stock and other seasonings, producing a broth that is powerful but balanced.
A bowl commonly contains rice vermicelli, pork, shrimp, squid, fish or roast pork, along with eggplant and a large plate of herbs and vegetables. Regional versions vary, especially in the choice of seafood and vegetables.
Bún mắm should not be confused with bún riêu, whose distinctive broth is based on crab and tomatoes. It is also different from bún bò Huế, whose defining profile comes from beef, lemongrass and chili rather than fermented fish.