Influenza-associated encephalopathy is rare in adults and the role of influenza virus in the pathogenesis of influenza-associated encephalopathy is unclear. We report a case of an adult patient who presented with typical clinical manifestations and magnetic resonance imaging findings of herpes simplex virus (HSV) encephalitis confirmed by positive PCR test in cerebrospinal fluid (CSF), and who was also PCR-positive PCR for H1N1 A/influenza in CSF and a nasopharyngeal swab. The results strongly suggest co-infection of the central nervous system. Given the significant implications for therapeutic interventions and infection control, A/H1N1 influenza should be considered one of the possible etiologies of viral encephalitis when patients present with an influenza-like illness during an influenza epidemic, even in those with typical manifestation of HSV encephalitis.