The energetic and mechanical stabilities of interstitial point defects in binary rock-salt materials were studied using the first-principles method. A novel, stable, and energetically competitive interstitial site (base-interstitial) was identified for anion interstitials in rock salts. The formation energies of base-interstitial defects were compared with well-explored tetrahedral (body-interstitial) and split interstitials and were found to be highly competitive energetically. For alkali halides and silver bromide, the lowest formation energies are associated with the base-interstitial site together with the <110> split interstitial, and these are therefore the predominant interstitial sites. However, in metal monochalcogenide systems, split interstitials were found to be the energetically preferred configuration. Electronic band structures are affected by the presence of interstitial defects in rock-salt structures. In particular, the Fermi level is shifted below the valence band maxima for the body, base, and split interstitials in metal halides, indicating p-type conductivity. However, the Fermi level remains within the bandgap for metal monochalcogenides, indicating no preferred conductivity for base- and split-interstitial defects. Allowing the defects to be charged changes the relative stability of the interstitial sites. However, for alkali halides, the new base-interstitial site remains preferred over a range of potentials. The discovery of a new interstitial site affects our understanding of defects in binary rock salts, including structure and dynamics as well as associated thermodynamic and kinetic properties that are interstitial dependent.