Green tea extracts effectively inactivated SARS-CoV-2 in vitro in a dose-dependent manner. Serially 10-fold diluted solutions of catechin mixture reagent from green tea were mixed with the viral culture fluid at a volume ratio of nine to one, respectively, and kept at room temperature for 5 min. The solution of 10 mg/mL catechin reagent reduced the viral titer by 4.2 log and 1.0 mg/mL solution reduced only by one log. Pre-infection treatment of the cells with the reagent alone did not affect the viral growth. In addition, cells treated with only the reagent was assayed for host-cell viability using the WST-8 system and almost no host-cell damage by the treatment was observed. These findings suggested that the direct treatment of virus with the reagent before inoculation decreased the viral activity and that catechins might have a potential to suppress the SARS-CoV-2 infection.