The present report describes a case of a patient with hepatitis B virus (HBV)-human immunodeficiency virus (HIV) co-infection who was treated with tenofovir disoproxil (TDF)-based highly active antiretroviral therapy (HAART) and who achieved HBs antigen (Ag)/antibody (Ab) seroconversion. An 18-year-old Japanese man with HIV and HBV co-infection presented to our hospital. His CD4 count was decreased, and TDF-based HARRT was started. At 30 months after initiation of therapy, HBsAg was not detected. At 36 months after initiation of therapy, HBsAb was detected. We conclude that TDF-based therapy is useful for the management of patients with HBV and HIV co-infection.