Oyster sauce is a viscous sauce prepared from oysters and brine, often with chemical preservatives added, commonly used in Chinese and Filipino cuisine. The sauce is often used as a topping for steamed vegetables such as kailan and in stir-fries.

Oyster sauce is said to be invented in 1888 by Mr. Lee Kam Sheung, in Nam Shui Village in Guangdong Province, China. His company, Lee Kum Kee, continues to produce oyster sauce, to this day, along with a wide variety of Asian condiments.

Vegetarian oyster sauce prepared from mushrooms is also popular and generally cheaper.

A "true" oyster sauce of good quality should be made by condensing oyster extracts (which is made by cooking oysters in water, where the aqueous mixture is then cooked until a desired viscosity has been reached) alone and no other additives except salt. The sauce should be aromatic with great flavour. Unfortunately this method is expensive and only a handful of manufacturers are willing to use it. Most oyster sauces are actually diluted solutions of concentrated ones.

Oyster sauce is capable of enhancing the flavour of other foods. A tablespoon of it makes plain Chinese noodles less ordinary.

Typical oyster sauce has monosodium glutamate added, but in recent years MSG-free varieties can also be found. The taste of MSG and non-MSG variants may be indistinguishable.