A metaborate is a borate anion consisting of boron and oxygen, with empirical formula . Metaborate also refers to any salt or ester of such anion (e.g. salts such as sodium metaborate  or calcium metaborate , and esters such as methyl metaborate ). Metaborate is one of the boron's oxyanions. Metaborates can be monomeric, oligomeric or polymeric.
In aqueous solutions metaborate anion hydrolyzes to tetrahydroxyborate . For this reason, solutions or hydrated salts of the latter are often improperly named "metaborates".