The transition metal-stabilized heavy main group radicals are extremely scarce due to their highly reactive natures, making them difficult to be isolated and identified. We report here a rare class of the Se radical-containing manganese carbonyl anionic cluster, [(μ-Se)(μ3-Se2)2Mn3(CO)9]•2– (1), which was successfully obtained from the one-pot reaction of Se powder and Mn2(CO)10 in concentrated KOH/MeOH/MeCN solutions at 90 °C. Dianion 1 and its dimeric cluster, [(μ4-Se2){(μ3-Se2)2Mn3(CO)9}2]4– [(1) 2 ], could undergo the reversible Se–Se bond breakage or reformation by the thermal cracking of (1) 2 or self-dimerization of 1, showing the μ-Se•– radical character of 1. Complex 1 could react with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to form the Se radical-captured complex [(μ-Se(TEMPO)) (μ3-Se2)2Mn3(CO)9]2– (1-TEMPO) or could react with alkylene bromides (CH2) n Br2 (n = 1, 2) to give the Mn4-based oxidative coupling products, [(μ4-Se2)(μ-Se2LSe)2Mn4(CO)12]2– (L = CH2, 2-CH 2 ; Se, 2-Se). In addition, dianion 1 and its aggregated derivatives (1) 2 , 1-TEMPO, 2-CH 2 , and 2-Se exhibited unusual paramagnetic properties with the spin-state switching from S = 1 (Mn) + 1/2 (Se) to S = 1 (Mn), in which their magnetic centers were proved to be mixed-valent Mn atoms and the μ-Se•– radical, as evidenced by Evans method, superconducting quantum interference device, X-ray photoelectron spectra, electron paramagnetic resonance, and density functional theory calculations. Importantly, these clusters showed semiconducting behaviors with low and tunable energy gaps (1.50–2.01 eV) and varied electrical conductivities (2.52 × 10–8–4.58 × 10–9 S/cm), where efficient electron transports mainly arose from C–H(phenyl)···O(carbonyl) interactions within the solid-state frameworks.