The (100) surface of single-crystal RuS2 was characterized using atomic force microscopy (AFM), scanning tunneling microscopy (STM), near-ddge X-ray absorption fine structure (NEXAFS), and electronic band structure calculations. High-resolution STM and AFM images show that the as-cleaved RuS2(100) surface has a square lattice with the same cell parameters as does bulk RuS2, but the unit cell features are distinctly differently from those expected for the unreconstructed surface. Tunneling spectra reveal that significant surface states are present within the band gap of bulk RuS2. Comparison of the electron and fluorescence yield NEXAFS analysis of the RuS2(100) surface indicates that the surface Ru atoms are bonded only to S atoms and exist in a bulklike valence state. Some of the surface S atoms are passivated with bound oxygen atoms. On the basis of electronic band structure calculations, we propose a model for the reconstructed RuS2(100) surface consistent with the experimental observations.