Priming Sugar is any sugar added to a fermented beer with the purpose of starting a secondary re-fermentation in a tank, a cask, a bottle, or more rarely, a keg. The end result is natural carbonation and additional flavor development. The sugar may be added as a solid, but it is most often added in liquid form just prior to racking the beer into a conditioning tank or its final container.

Yeast, which may be added at the same time as the priming sugar, consumes the priming sugars, giving off carbon dioxide and carbonating the beer inside the tank or package. Priming sugars are usually highly fermentable, with the most common being neutrally flavored sucrose, glucose, and dextrose. Under the correct conditions, these sugars will ferment out completely and leave no extra residual sweetness in the finished beer. Other sugars, such as invert sugar and molasses, although rarely used, can contribute their own colors and flavors to a finished beer.

In British cask-conditioned beers, which have a low, but brisk natural carbonation, the addition of priming sugar is usually 1.5 g to 3.0 g/l. The more priming sugar added, the more carbonation is likely to be produced, with some vigorously carbonated Belgian specialty beers using up to 14 g of priming sugar per liter.