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We report the experimental investigation of Mn doping effect on the electrical, magnetic, and thermal properties of Ru _1− _x Mn _x Sb _2+ _δ single crystals. While RuSb _2+ _δ is a diamagnetic semiconductor, partial replacement of Ru by Mn results in ferromagnetism with the Curie temperature 536 K and 540 K for x  = 0.04 and 0.08, respectively. Correspondingly, the electrical resistivity decreases dramatically and shows metallic character due to the increased electron concentration. Surprisingly, the magnitude of the thermopower increases upon Mn doping, reaching −250 μ V K ^−1 at 300 K for x  = 0.08, while the thermal conductivity remains unchanged above 100 K. These indicate a strong modification of electronic structure by doped Mn, leading to enhanced thermoelectric properties. The carriers induced by Mn not only decrease the resistivity, but also mediate the long-range ferromagnetic ordering in Ru _1− _x Mn _x Sb _2+ _δ .