The present study investigated the effect of tea catechins on the immune function in elderly subjects using a catechin-rich beverage. Twenty Japanese individuals aged over 65 years participated in the study after written informed consent was obtained. The present single-arm pilot study was conducted for an intervention period of two weeks, after one week of abstinence from consuming tea/catechin-rich products. Participants consumed one bottle of the catechin-rich beverage (total catechins, 540 mg in 350 mL) every morning. The participants' immune function including the change in natural killer (NK) cell activity (％) from before to after intervention were evaluated. Pearson's correlation coefficient (r) was calculated to investigate the correlation between NK cell activity before intervention and change in NK cell activity. The study participants appropriately consumed the catechin-rich beverage every day without any adverse event. The mean NK cell activity at baseline was 41.7 (standard deviation, 23.1). After the intervention, NK cell activity increased in 17 participants (85.0％), and the estimated mean of the differences indicated a statistically significant increase (17.1; 95％ confidence interval, 9.7 to 24.5). No significant changes in C-reactive protein and interleukin-6 levels were observed. Moreover, pre-intervention NK cell activity correlated significantly with the ratio of post- to pre-intervention NK activity (r＝−0.63). Although further studies using a parallel-group and comparative design are required to clarify the effect of tea catechins on the immune function, our results suggest that catechin consumption may enhance or maintain NK cell activity in the elderly.