In this paper, we report room-temperature ferromagnetism in chemically synthesized Zn1-x Co (x) S (0 a parts per thousand currency sign x a parts per thousand currency sign 0.10) diluted magnetic semiconductor nanoparticles of 3-5 nm. The incorporation of Co2+ ion for Zn2+ ions in ZnS lattice and the particle size were confirmed by XRD and TEM along with selected area electron diffraction analysis. UV-Vis measurement showed reduction in the bandgap energy with the increase in Co doping. Maximum magnetization was observed for samples with x = 0.04. From photoluminescence, spectra luminescence efficiency was found to get enhanced on Co doping. Magnetization behavior can be understood to be due to defect-induced ferromagnetism; however, for higher doping concentration, the antiferromagnetic coupling of Co-Co interaction in the close proximity results in the decrease of the overall magnetization of the samples. Moreover, magneto-electronic study also showed a maximum negative magneto-resistance of 43 % for x = 0.04.