Ribosome-inactivating proteins, including Saporin toxin, have found application in the search for innovative alternative cancer therapies to conventional chemo- and radiotherapy. Saporin’s main mechanism of action involves the inhibition of cytoplasmic protein synthesis. Its strong theoretical efficacy is counterbalanced by negligible cell uptake and diffusion into the cytosol. In this work, we demonstrate that by immobilizing Saporin on iron oxide nanoparticles coated with an amphiphilic polymer, which promotes nanoconjugate endosomal escape, a strong cytotoxic effect mediated by ribosomal functional inactivation can be achieved. Cancer cell death was mediated by apoptosis dependent on nanoparticle concentration but independent of surface ligand density. The cytotoxic activity of Saporin-conjugated colloidal nanoparticles proved to be selective against three different cancer cell lines in comparison with healthy fibroblasts.