​​ ​In this study, boron-doped cadmium sulfide nanoparticles (B-doped CdS NPs) with varying boron concentrations were synthesized by a hydrothermal method for evaluation as potential anticancer agents. The effects of boron doping on the structural, thermal and physicochemical properties of CdS NPs were characterized comprehensively. X-ray diffraction confirmed a hexagonal crystal structure for all B-doped samples. Scanning electron microscopy showed particles in the 30-90 nm range that aggregated into petal-like morphologies. Thermogravimetric analysis and zeta-potential measurements indicated improved colloidal/thermal stability after doping: total mass loss observed in TGA was substantially reduced, and the surface potential magnitude increased from approximately -18.9 mV to -30 mV. Cytotoxicity was assessed by exposing normal human dermal fibroblasts (HDF) and two cancer cell lines (MCF-7 breast and A549 lung) to the B-doped CdS NPs. IC50 values demonstrate dose-dependent cytotoxic effects that are substantially stronger in cancer cells than in normal cells, with anticancer activity increasing with higher boron doping levels. These results indicate that B-doped CdS NPs warrant further investigation as selectively active nanoparticles for biomedical applications.​