The manganese-nitronyl-nitroxide two dimensional coordination polymer {[Mn₂(NITIm)₃]ClO₄}_n (1) (NITImH = 2-(2-imidazolyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxide-1-oxyl) undergoes an unusual hysteretic thermoinduced valence tautomeric transition near room temperature, during which the manganese(II) ions are oxidized to manganese(III) and two of the three deprotonated radicals (NITIm-) are reduced to their diamagnetic aminoxyl form (denoted NITIm_Red^2-). Upon cooling, the high-temperature species {[Mn^II₂(NITIm)₃]ClO₄}_n (1_HT) turns into the low-temperature species {[Mn^III₂(NITIm_Red)₂(NITIm)]ClO₄}_n (1_LT) around 274 K, while on heating the process is reversed at about 287 K. This valence tautomeric phenomenon is supported by temperature-dependent magnetic susceptibility measurements, differential scanning calorimetry (DSC), crystal structure determination, UV-vis absorption, X-ray absorption (XAS) an emission (XES) and Electron Paramagnetic Resonance (EPR) spectroscopies in the solid-state.