The fractional oxidation state [M(dmit)2] (dmit2- = 2-thioxo-1, 3-dithiole-4, 5-dithiolate) salts have long attracted attention in the molecular metal area owing to high conductivity and even superconductivity. In this study, we achieved a mixed-valence salt (1) of [Ni(dmit)2]0.5- with monovalent 1,3-N,N-dimethyl-imidazolium (DiMIm+) by a solvent evaporation approach under ambient conditions. The mixed valence of [Ni(dmit)2]0.5- has been characterized by an analysis of the IR spectrum and crystal structure. In the crystal structure of 1, two [Ni(dmit)2]0.5- anions overlap in an eclipsed mode to form a [Ni(dmit)2]21- dimer, featuring a radical bearing an S = 1/2 spin; the dimeric radicals stack into a column along the b axis, and the adjacent columns connect together via the lateral-to-lateral S···S contacts along the a axis, and through the head-to-head S···S contacts along the [101] direction. Salt 1 shows the magnetic behavior of an S = 1/2 Heisenberg antiferromagnetic uniform linear chain with J/kB = -47.5(4) K and a semiconducting feature with σ = 2.52 × 10-3 S cm-1 at 293 K, 2.32 × 10-2 S cm-1 at 373 K, and Ea = 0.22 eV, as well as broadband photoconductivity under irradiation of green and white lights. This study suggests the possibility of designing new photoconductors based on the mixed-valence [Ni(dmit)2]0.5- salt.