Gueuze (or Geuze) is a type of lambic, a Belgian beer. It is made by blending young (1-year-old) and old (2- to 3-year-old) lambics into a new beer, which is then bottled for a second fermentation. Because the young lambic is not fully fermented, it contains fermentable sugars, which allow the second fermentation to occur. Lambic that undergoes a second fermentation in the presence of sour cherries before bottling results in kriek lambic, a beer closely related to gueuze.

Since gueuze is made by blending lambics, it tastes different from traditional ale and lager style beers. Because aged hops are used to produce these lambics, the beer has little to none of the traditional hop flavor and aroma that can be found in most other styles of beer. Furthermore, the wild yeasts that are specific to lambic-style beers give gueuze a dry, cidery, musty, sour, acetic acid, lactic acid taste. Many describe the taste as sour and "barnyard-like." In modern times, some brewers have added sugar to their gueuzes to sweeten them and make the beer more appealing to a wider audience. Because of its carbonation, gueuze is sometimes called "Brussels Champagne."

Traditionally, gueuze is served in champagne bottles, which hold either 375 millilitres (12.7 US fl oz) or 750 millilitres (25 US fl oz).