The str, ucture of the HO3- anion has been studied theoretically using many theoretical methods. We found two metastable isomers for HO3-. The most stable isomer has a structure reminiscent to its neutral HOOO and cationic HOOO+ relatives. However, the central O-O bond in the anion was found to be unusually long (1.79 Angstrom) compared to the same bond in the cation (1.40 Angstrom) and in the neutral molecule (1.56 Angstrom). Careful analysis reveals that the long O-O bond in the anion is a true chemical bond, but the bond order is only 0.75, which is the reason for its extreme length. We believe that the HO3- anion may play a role in atmospheric processes and its unusual long bond can expand our view of chemical bonding in oxygen-rich molecules.