What fermented fish sauce is used in Pad Thai?

The fermented fish sauce used in Pad Thai is fish sauce. In Thai, the everyday condiment is called nam pla, and it supplies saltiness as well as a deep savory umami flavor to the noodle dish's sauce.

Fish sauce is generally produced by fermenting fish with salt for an extended period, then extracting and filtering the resulting liquid. Anchovies are especially common in commercial fish sauces, although the fish and production methods vary. The finished condiment is thin and aromatic, quite different from thicker oyster sauce.

Pad Thai sauce usually balances fish sauce with a sweetener and a souring ingredient such as tamarind. Vegetarian versions may replace fish sauce with soy sauce or another plant-based seasoning, but that is a variation rather than the standard preparation. Worcestershire sauce can contain fermented ingredients, yet it is a Western condiment and is not the canonical answer for Pad Thai.