In biochemistry, a compound that has been denatured has lost its native state, or in other words, no longer has the shape that is most compact and allows for optimal biological activity. In proteins, a denatured protein has no secondary structure other than random coil. A classic example of denaturing in proteins comes from the whites of eggs, which are largely egg albumens. Fresh from the eggs, egg whites are transparent and flow. But by cooking they are turned opaque and white, and form an interconnected solid mass. The same transformation can be effected with a denaturing chemical. Pouring egg whites into a beaker of acetone will also turn egg whites opaque and solid.

In many proteins (unlike egg whites), denaturation is reversible. This was important historically, because it led to the notion that all the information needed for proteins to assume their native state was encoded in the primary structure of the protein, and hence in the DNA that codes for the protein.

In the United States, "denatured" has a specific commercial meaning when applied to alcohol. Denatured alcohol is ethanol that has been made unfit for human consumption by the addition of harmful additives in order to make it exempt from taxes that apply to potable alcohol. (See: Denatured alcohol.)