Acid phosphatase is a phosphatase used to free attached phosphate groups from other molecules during digestion. It is stored in lysosomes and functions when these fuse with endosomes, which are acidified while they function; therefore, it has an acid pH optimum.

Different forms of acid phosphatase are found in different organs, and their serum levels are used as a diagnostic for disease in the corresponding organs. For example, elevated prostatic acid phosphatase levels may indicate the presence of prostate cancer.