It is estimated that 4 to 5 million people are currently co-infected with Human Immunodeficiency Virus (HIV) and Hepatitis C Virus (HCV). HIV/HCV co-infection is associated with unique health risks including increased hepatotoxicity of antiretrovirals, accelerated progression of HCV and liver diseases. The standard interferon-based therapy is effective only in about 50% of patients and often is associated with autoimmune and neuro-psychiatric complications. The treatment of co-infection (HIV/HCV) requires new strategic approaches. To this end, the formulations of an amphiphatic α-helical peptide, a positively charged analog of C5A peptide derived from the HCV NS5A protein, with a reported virocidal activity were prepared by electrostatic coupling with anionic poly(amino acid)-based block copolymers. The self-assembled antiviral peptide nanocomplexes (APN) were ca. 35 nm in size, stable at physiological pH and ionic strength, and retained in vitro antiviral activity against HCV and HIV. Moreover, incorporation of the peptide into APN attenuated its cytotoxicity associated with the positive charge. In vivo APN were able to decrease the viral load in mice transplanted with human lymphocytes and HIV-1-infected. Overall, these findings indicate the potential of these formulations for stabilization and delivery of antiviral peptides while maintaining their functional activity. Published by Elsevier Ltd.