A phosphatase is an enzyme that hydrolyses phosphoric acid monoesters into a phosphate ion and a molecule with a free hydroxy group. This action is directly opposite to that of phosphorylases and kinases, which attach phosphate groups to their substrates at the expense of energy like ATP. A common phosphorylase in the body is alkaline phosphatase.

The presence or absence of the phosphate group on proteins, especially enzymes, is known to play a regulatory role in many biochemical pathways and signal transduction pathways. Hence together, specialized kinases and phosphatases regulate enzymatic activity.