A 23-year-old man was admitted to our hospital with severe thrombocytopaenia. He had unprotected sexual contact 6 weeks earlier. He was diagnosed with acute HIV infection by means of HIV RNA viral load testing and HIV-associated thrombocytopaenia. Although his thrombocytopaenia improved immediately with short-term dexamethasone therapy, this effect was not sustained after cessation of therapy. Antiretroviral therapy including raltegravir was initiated, and the patient recovered from severe thrombocytopaenia within several days. The findings from this case suggest that acute HIV infection should be suspected with unexplained thrombocytopaenia, and that antiretroviral therapy is the treatment of choice for severe HIV-associated thrombocytopaenia, even when in the early period following acquisition of the virus.