A metaphosphate ion is an oxyanion that has the empirical formula PO₃^-. The structure of a metaphosphate ion can be described as being made up of PO₄ structural units in which each unit shares two corners with another unit. This can come about in two ways.

• Formation of a ring, as in trimetaphosphate, illustrated.
• Formation of an infinite chain, with the same structure as in ammonium metavanadate