Vertically-aligned Mn (10%)-doped Fe3O4 (Fe2.7Mn0.3O4) nanowire arrays were produced by the reduction/substitution of pre-grown Fe2O3 nanowires. These nanowires were ferromagnetic with a Verwey temperature of 129 K. X-ray magnetic circular dichroism measurements revealed that the Mn2+ ions preferentially occupy the tetrahedral sites, substituting for the Fe3+ ions. We observed that the Mn substitution decreases the magnetization, but increases the electrical conductivity. We developed highly sensitive gas sensors using these nanowire arrays, operating at room temperature, whose sensitivity showed a correlation with their bond strength of diatomic/triatomic molecules. Based on the fact that the sensitivity was highest toward water vapor, an excellent-performance humidity sensor was fabricated.