A material's proof stress is the maximum amount of stress it can withstand before undergoing a small bit of plastic deformation. Proof stress is defined as the point where the material demonstrates 0.2 percent plastic deformation.

This type of stress is commonly employed in the industrial business to guarantee that a material's elastic limit is not exceeded. That is, a safe load is provided to the object so that it is not stressed to the point of irreversible size or shape change. If a material is allowed to surpass its proof stress, it might permanently distort, leading to failure or fracture.

In the maritime and offshore industries, proof stress is particularly useful. The proof stress of ships and platform legs is crucial in evaluating how much force these structures can withstand before permanently deforming.

Because it is the stress required to make the material yield, proof stress is also known as offset stress.