First-principles molecular orbital calculations for the electronic states of hydrogen in ZrO 2 have been carried out using the discrete variational-Xα cluster method. Both the band gap energy and the valence band structure of ZrO 2 are in good agreement with experimental results. By the introduction of a hydrogen atom, a new impurity level appears below the conduction band and the band gap is greatly reduced. Calculated partial density of states of the H 1s component in the band gap reproduces the experimental results obtained by means of Mott–Schottky analysis. The dominant chemical bond in ZrO 2 is found to be the strong ionic bond between Zr and O atoms, and the bonding character is hardly modified by the hydrogen content.