Oxide-ion conductors based on hexagonal perovskite-related oxide Ba7Nb4MoO20 have attracted much attention due to high oxide-ion and proton conductivities and potential applications in many electrochemical devices such as solid oxide fuel cells (SOFCs). Herein, we report simultaneous improvement of oxide-ion conductivity and suppression of proton conductivity by Cr6+ doping in Ba7Nb4MoO20. New materials Ba7Nb4‒xMoCrxO20+x/2 (x = 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5) were synthesized. It was found that Ba7Nb3.8Cr0.2MoO20.1 exhibits high oxide-ion conductivity of 1.6 × 10‒3 S cm‒1 at 508 ℃ and 1.1 × 10‒2 S cm‒1 at 904 ℃ in static air and low proton transport number under wet conditions. Ba7Nb3.8Cr0.2MoO20.1 also shows wide electrolyte domain in the oxygen partial pressure P(O2) regions from 1 to 2.2 × 10‒27 atm (304 ℃) and 1 to 1.5 × 10‒26 atm (604 ℃), indicating extremely high chemical and electrical stability. The structure analyses have shown that Ba7Nb3.8Cr0.2MoO20.1 is a hexagonal perovskite-related oxide at 22 and 800 °C. Refined crystal structure of Ba7Nb3.8Cr0.2MoO20.1 has oxygen-deficient cubic (c′) close-packed Ba(O1)2−y(O5)z layer where y is the amount of oxygen vacancy at the tetrahedral O1 site and z is the amount of interstitial octahedral oxygen atoms at an O5 site. The bond valence sum of Cr cation indicates the oxidation number of + 6: Cr6+. The Cr6+ cation is located at a crystallographic site near the c′ layer, which leads to the excess oxygen and high conductivity, and is likely to suppress the proton conduction. Maximum-entropy method (MEM) analyses have demonstrated that oxide ions two-dimensionally migrate through the lattice O1 and interstitial O5 sites in the c′ layer via the interstitialcy diffusion mechanism at 800 °C, which enables the high oxide-ion conduction. The Cr6+ doping in various hexagonal perovskite-related oxides would be a new strategy for the simultaneous improvement of oxide-ion conductivity and suppression of proton conduction.