In the present study, solid lipid nanoparticles (SLNs) have been formulated as a carrier system for effective intracellular delivery of STAT3 inhibitor, niclosamide (Niclo) to triple negative breast cancer (TNBC) cells. Emulsification-solvent evaporation method was employed in formulation of Niclo-loaded SLNs (Niclo-SLNs). The formula of Niclo-SLN was optimized by Box-Behnken design and characterized for their shape, size, and surface charge. The in vitro anti-cancer efficacy of Niclo-SLNs was studied in TNBC cells. The prepared Niclo-SLNs were found to be spherical with the particle size of 112.18 ± 1.73 nm and zetapotential of 23.8 ± 2.7 mV. In the in vitro anticancer study the Niclo SLNs show a better cytotoxicity than the naïve Niclo, which is attributed to improved cell uptake of SLN formulation. In conclusion, the results of the present study demonstrate that the formulation of Niclo as SLNs will improve the anticancer efficacy against TNBC.