Nonporous silica nanoparticles (SNPs) with an external shell containing primary amino groups were proposed as potential delivery systems for Pt(iv) antitumor prodrugs. Spherical SNPs containing two different external arms, i.e. 3-aminopropyl and N-(6-aminohexyl)aminomethylene, of around 125 nm hydrodynamic diameter were loaded with two different cisplatin-based Pt(iv) complexes, namely (OC-6-44)-diamminedichloridoethoxidosuccinatoplatinum(iv) and (OC-6-44)-diamminedichloridoacetylamidosuccinatoplatinum(iv), through the formation of amide bonds between the pendant amino groups on SNPs and the free carboxylic group of the complexes. In the presence of the N-(6-aminohexyl)aminomethylene arm, the Pt(iv)-SNP conjugates showed a negligible (unwanted) Pt release by hydrolysis, whereas in the presence of ascorbic acid the reduction of Pt(iv) → Pt(ii) caused the substantial release of the active metabolite cisplatin. Conjugate Pt(iv)-SNP exhibited better antiproliferative activity on the Pt-sensitive A2780 human ovarian cancer cell line than the parent cisplatin and their free Pt(iv) precursors, due to their more efficient cellular uptake, likely by endocytosis.