BACKGROUND Dermal fillers are increasingly used for medical and aesthetic purposes in clinical practice. Common complications following filler injections include bruising, itching, infections, allergic reactions, and tissue necrosis. This case is the first report of Herpes simplex virus type 1 (HSV-1) encephalitis as a possible complication of dermal filler injection. CASE REPORT A 27-year-old woman with no past medical history presented with altered mental state, headaches, and seizures. She had a nasal dermal filler injection for aesthetic purpose five weeks before her acute presentation. A diagnosis of HSV-1 encephalitis was made based on brain imaging with computed tomography and magnetic resonance imaging (MRI) findings that showed bilateral frontotemporal lobe hyperintensity. Analysis of her cerebrospinal fluid (CSF) confirmed the presence of HSV-1 DNA. Despite anti-viral treatment with acyclovir, she developed postencephalitic syndrome. CONCLUSIONS This case report highlights the possibility that among the complications of the use of cosmetic dermal fillers, the transmission of HSV-1 and the development of HSV-1 encephalitis should be recognized.