In chemistry a reactive intermediate is a short-lived high energy highly reactive molecule. When generated in a chemical reaction it will quickly convert into a more stable molecule. Only in exceptional cases these compounds can be isolated and stored. Reactive intermediates help explain how a chemical reaction takes place.

Most chemical reactions take more than one elementary step to complete, and an reactive intermediate is an unstable product that only exists in one of the intermediate steps. The series of steps together make a reaction mechanism. It differs from the reactants and products in the way that it cannot be isolated, and is thus not directly observable. An example would be

A + 2B → C + D + E,