We grew large single crystals of the cluster magnet Nb3CI8 with a magnetic triangular lattice and investigated its magnetic properties and crystal structure. In Nb3Cl8, the [Nb-3](8+) cluster has a single unpaired spin, making it an S = 1/2 triangular lattice antiferromagnet. At low temperatures, Nb3CI8 exhibits a magnetic nomnagnetic phase transition driven by a charge disproportionation, in which the paramagnetic [Nb-3](8+) clusters transform into alternating layers of nonmagnetic [Nb-3](7+) and [Nb-3](9+) clusters. The observed exotic phenomenon with the strong correlation between the magnetism and structure are based on the nature of the cluster magnetism.