Royal icing is a hard white icing, made from softly beaten egg whites, icing sugar (powdered sugar), and sometimes lemon or lime juice. It is used to decorate Christmas cakes, wedding cakes, gingerbread houses, cookies and many other cakes and biscuits. It is used either as a smooth covering or in sharp peaks. Glycerine is often added to prevent the icing from setting too hard. When placing icing on cakes, marzipan is usually used under the royal icing in order to prevent discoloration of the icing.

Usual proportions are 2 egg whites to 1 teaspoon of lemon juice, 1 teaspoon of glycerine, and around 1 pound of sugar depending on the application.

As well as coating cakes and biscuits, royal icing is used to make decorations such as flowers and figures for putting on the cake. The royal icing is piped into shapes which are allowed to harden on a non-stick surface. These can then be arranged to create edible decorative effects on a variety of sweet foods. The glycerine must be omitted for this purpose. Royal icing is often used to create snow scenes but is also used as an edible adhesive – particularly for gingerbread houses.