Cu2ZnSnS4 (CZTS) nanoparticles were synthesized by the precursor injection method using oleylamine as a solvent. Preliminary characterization indicated that the synthesized nanoparticles belonged to the kesterite structure with a bulged sphere-like morphology. Reduced graphene oxide (rGO) was synthesized by an improved Hummers method and was used for nanoparticle functionalization. CZTS nanocrystals were decorated on rGO by two different methods. One was oleylamine-based nanoparticle functionalization, and the other was in situ nanoparticle growth. Transmission electron microscopy analysis of CZTS-functionalized rGO showed that the synthesized nanoparticles were uniformly spread on the surface of rGO sheets. Single phase CZTS nanoparticles were grown on rGO without any impurity phase in the in situ growth. Tuned absorption of the pure CZTS was observed by the decoration of CZTS nanoparticles on the surface of rGO in the visible and UV regions.