The absolute hydration free energy of the hydroxide ion, DeltaG(hyd)(298)(HO-), a fundamental quantity in solution chemistry, has "experimental" values ranging from -90.6 to -110.0 kcal/mol. We report a first-principles determination of DeltaG(hyd)(298)(HO-) by using a reliable computational protocol of high-level first-principles supermolecule-continuum calculations, the same approach recently used to determine the absolute hydration free energy of the proton. In the supermolecule-continuum approach, part of the solvent surrounding the solute is treated quantum mechanically, and the remaining bulk solvent is approximated by a dielectric continuum medium accounted for by a recently developed self-consistent reaction field model known as surface and volume polarization for electrostatic interaction (SVPE) or the. fully polarizable continuum model (FPCM). With this approach, the calculated results can systematically be improved by increasing the number of quantum mechanically treated solvent molecules, and DeltaG(hyd)(298)(HO-) is accurately predicted to be -104.5 kcal/mol. The DeltaG(hyd)(298)(HO-) value of -104.5 kcal/mol, combined with our previously determined DeltaG(hyd)(298)(H+) value of -262.4 kcal/mol, allows the prediction of the sum of absolute hydration free energies of the proton and hydroxide to be -366.9 kcal/mol, in excellent agreement with the well-established experimental thermodynamic value of -366.6 +/- 0.1 kcal/mol.