Metaboric acid is the name for a family of inorganic compounds with the same empirical formula HBO2[2] that differ in their molecular structure.

They are colourless water-soluble solids formed by the dehydration or decomposition of boric acid.

Metaboric acid is formally the parent acid of the metaborate anions.

The main forms of metaboric acid are: When heated above about 170 °C, metaboric acid dehydrates, forming tetraboric acid, also called pyroboric acid (H2B4O7):[6][7] Metaborates are derivatives of BO2−.

Examples are sodium and potassium metaborates, salts formed by deprotonation of orthorhombic metaboric acid containing the cyclic B3O63− ion and calcium metaborate, Ca(BO2)2, which contains the chain polymeric ion (BO2−)n.[2]