In physics and chemistry, vitreous ice is a difficult-to-achieve state of water formed by cooling it extremely rapidly in which crystallization does not occur.

Generally, when water is cooled below the conventional freezing temperature, it forms crystalline ice. This is responsible for the well known fact that water expands as it freezes. However, if water is cooled extremely rapidly, such as by rapidly plunging it into a liquid nitrogen slush, liquid ethane, etc., it freezes so rapidly that crystallization does not occur. This state is only stable at very low temperatures, so it is not commonly observed in nature. However, this state is widely used in scientific experiments, for example, electron cryomicroscopy where individual molecules can be preserved for imaging.