Platinum(IV) complexes take advantage of the exclusive conditions that occur within the tumor to carry out their cytotoxic activity. On the other hand, silk fibroin has natural properties which make it very interesting as a biomaterial: highly biocompatible, biodegradable, low immunogenicity, high cellular penetration capacity and high reactive surface. Herein we report the preparation of silk fibroin nanoparticles (SFNs) loaded with the hydrophobic Pt(IV) complex cis,cis,trans-[Pt(NH3)2Cl2(O2CC6H5)2] (PtBz). Only a small fraction of the loaded PtBz is released (less than 10% after 48 h). PtBz-SFNs trigger strong cytotoxic effects against human ovarian carcinoma A2780 cells and its cisplatin-resistant variant A2780cisR cells. Interestingly, PtBz-SFNs are very cytotoxic (nanomolar IC50 values) toward the triple negative breast tumor cell line MDA-MB-231, and also toward SK-BR-3 and MCF-7, while maintaining an excellent selectivity index.