Ultrapure water (UPW), high-purity water, or highly purified water (HPW) is water that has been purified to uncommonly stringent specifications.

Ultrapure water is a term commonly used in manufacturing to emphasize the fact that the water is treated to the highest levels of purity for all contaminant types, including organic and inorganic compounds; dissolved and particulate matter; volatile and non-volatile; reactive, and inert; hydrophilic and hydrophobic; and dissolved gases.