Fish sauce adds the savory taste called umami to dishes like Pad Thai.
Fish sauce is made by fermenting fish or krill with salt, often for many months and sometimes up to two years. During fermentation, proteins break down and produce flavor compounds including glutamate, which contributes strongly to umami.
Umami is one of the five basic tastes, alongside sweet, sour, salty, and bitter. It is often described as savory or brothy and is associated with foods such as cooked meat, mushrooms, aged cheese, soy sauce, and fermented fish products.
Fish sauce also adds saltiness, so umami and salty are not interchangeable descriptions. A small amount can deepen a dish without making it taste overtly fishy, especially when balanced with tamarind, sugar, chili, or lime.
The taste has a long culinary history. Fermented fish sauces were used in ancient Mediterranean cooking, including Roman garum, while modern fish sauce is especially central to Southeast Asian cuisines such as Thai and Vietnamese cooking.