A metaborate is a borate anion consisting of boron and oxygen, with empirical formula BO−2.

For this reason, solutions or hydrated salts of the latter are often improperly named "metaborates".

In the solid state of their salts, metaborate ions are often oligomeric or polymeric, conceptually resulting from the fusion of two or more BO−2 through shared oxygen atoms.

The vapor of cesium metaborate has neutral monomers CsBO2 and dimers Cs2[B2O4] as well as ionized versions thereof.

[19] In 1964 Hisatsune and Surez investigated the infrared spectrum of metaborate anions in dilute solid solutions of potassium salt in alkali halides such as potassium chloride KCl.