Human T-cell lymphotropic virus type 1 (HTLV-1) is causatively associated with adult T-cell leukaemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Since a high level of HTLV-1 provirus load in circulating lymphocytes is thought to be a risk for ATL and HAM/TSP, diminution of HTLV-1 provirus load in the circulation may prevent these intractable diseases. A previous study demonstrated that Camellia sinensis (green tea) polyphenols inhibit in vitro growth of ATL cells, as well as HTLV-1-infected T-cells. The present study aimed to investigate the in vivo effect of C. sinensis polyphenols on HTLV-1 provirus load in peripheral blood lymphocytes on HTLV-1 carriers. Eighty-three asymptomatic HTLV-1 carriers were recruited to examine HTLV-1 provirus DNA with or without administration of capsulated C. sinensis extract powder. Thirty-seven subjects were followed up for 5 months by measuring HTLV-1 provirus load after daily intake of nine capsules of C. sinensis extract powder per day (equivalent to 10 cups of regular green tea), and 46 subjects lived ad libitum without intake of any C. sinensis capsule. The real-time PCR quantification of HTLV-1 DNA revealed a wide range of variation of HTLV-1 provirus load among asymptomatic HTLV-1 carriers (0.2–200.2 copies of HTLV-1 provirus load per 1000 peripheral blood lymphocytes). Daily intake of the capsulated C. sinensis for 5 months significantly diminished the HTLV-1 provirus load as compared with the controls.