In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid, aka. phosphoric acid H₃PO₄.

The phosphate or orthophosphate ion [PO
₄]³⁻
is derived from phosphoric acid by the removal of three protons H⁺
. Removal of one or two protons gives the dihydrogen phosphate ion [H
₂PO
₄]⁻
and the hydrogen phosphate ion [HPO
₄]²⁻
ion, respectively. These names are also used for salts of those anions, such as ammonium dihydrogen phosphate and trisodium phosphate.

In organic chemistry, phosphate or orthophosphate is an organophosphate, an ester of orthophosphoric acid of the form PO
₄RR′R″ where one or more hydrogen atoms are replaced by organic groups. An example is trimethyl phosphate, (CH
₃)
₃PO
₄. The term also refers to the trivalent functional group OP(O-)
₃ in such esters.

Orthophosphates are especially important among the various phosphates because of their key roles in biochemistry, biogeochemistry, and ecology, and their economic importance for agriculture and industry. The addition and removal of phosphate groups (phosphorylation and dephosphorylation) are key steps in cell metabolism.

Orthophosphates can condense to form pyrophosphates.