The present invention is related to the pharmaceutical field, and particularly to a pharmaceutical composition for intratumoral injection comprising nanoparticles of natural origin (chitosan), which are used as vectors for encapsulation of genes for proteins that are able to control tumour growth, generate an immune response, or both. In particular, the pharmaceutical composition for intratumoral injection comprises chitosan nanoparticles encapsulating DNA from the fusion protein from avian reovirus (REO), which slows tumour growth and finally leads to elimination thereof by generation of syncytia, and the use thereof in cancer treatment by in vivo transfection.