Mucopolysaccharidosis type IIIA (MPS-IIIA) or Sanfilippo A syndrome is a lysosomal storage genetic disease that results from the deficiency of the N-sulfoglucosamine sulfohydrolase (SGSH) protein, a sulfamidase required for the degradation of heparan sulfate glycosaminoglycans (GAGs). The accumulation of these macromolecules leads to somatic organ pathologies, severe neurodegeneration and death. To assess a novel gene therapy approach based on prolonged secretion of the missing enzyme by the liver, mediated by hydrodynamic gene delivery, we first compared a kanamycin and an antibiotic-free expression plasmid vector, called pFAR4. Thanks to the reduced vector size, pFAR4 derivatives containing either a ubiquitous or a liver-specific promoter mediated a higher reporter gene expression level than the control plasmid. Hydrodynamic delivery of SGSH-encoding pFAR4 into MPS-IIIA diseased mice led to high serum levels of sulfamidase protein that was efficiently taken up by neighboring organs, as shown by the correction of GAG accumulation. A similar reduction in GAG content was also observed in the brain, at early stages of the disease. Thus, this study contributes to the effort towards the development of novel biosafe non-viral gene vectors for therapeutic protein expression in the liver, and represents a first step towards an alternative gene therapy approach for the MPS-IIIA disease.