A 19-month-old female common marmoset (Callithrix jacchus) was presented with a 4-day history of anorexia and behavioral changes. At presentation, a combination of neurological signs, erosions, and ulcerations of the mucous membranes of the oral mucosa and mucocutaneous junction of the lips were observed, suggesting herpesvirus infection. Although etiological and symptomatic therapy was initiated immediately, the animal died the next day. A necropsy was performed, and gross pathology and pathohistology findings indicated a systemic viral infection. Definitive diagnosis was based on the results of molecular testing that demonstrated the presence of human herpes simplex virus type 1. In conclusion, marmosets are a highly susceptible host for human herpes simplex virus. To reduce the risk of infection and prevent this highly lethal disease in monkeys, contact with humans who have symptomatic or asymptomatic forms of HSV-1 should be limited and accompanied by appropriate hygiene measures.