We report the synthesis of a semiquinoid-bridged single-chain magnet, as generated through a thermally induced metal–ligand electron transfer. Reaction of FeCl₃ with 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH₂) in the presence of (NMe₄)Cl gave the compound (NMe₄)₂[LFeCl₂]. Together, variable-temperature X-ray diffraction, Mössbauer spectra, Raman spectra, and dc magnetic susceptibility reveal a transition from a chain containing (L^2–)Fe^II units to one with (L^3–•)Fe^III upon decreasing temperature, with a transition temperature of T_1/2 = 213 K. The dc magnetic susceptibility measurements show strong metal–radical coupling within the chain, with a coupling constant of J = −81 cm^–1, and ac susceptibility data reveal slow magnetic relaxation, with a relaxation barrier of Δ_τ = 55(1) cm^–1. To our knowledge, this compound provides the first example of a semiquinoid-bridged single-chain magnet.