Gastrique is caramelized sugar, deglazed with vinegar or other sour liquids, used as a sweet and sour flavoring for sauces.

The gastrique is generally added to a fond, reduced stock or brown sauce. It is also used to flavor sauces such as tomato sauce, savory fruit sauces, and others, such as the orange sauce for duck à l'orange.

The term is often broadened to mean any sweet and sour sauce, e.g., citrus gastrique or mango gastrique. An agrodolce is a similar sauce found in Italian cuisine.

It is different from the Belgian sauce base of the same name, which consists of vinegar, white wine, shallots, tarragon stems, bouquet garni, and peppercorns. The gastrique with this composition was already used by Auguste Escoffier, but at the end of the 19th century, Louis Védy from Brussels turned it into a plant extract that ensures a constant level of acidity when making béarnaise sauce.