Green tea catechins are well known to be one of polyphenols, and its regular ingestion induces body fat reduction in obese individuals. Cocoa polyphenols of high-cocoa chocolate can also improve arterial stiffness. However, it is unclear whether green tea catechins improve body fat and arterial stiffness even in healthy young adults. Therefore, this study aimed to examine the effects of regular green tea intake on body fat and arterial stiffness in young adults. This randomized, controlled, parallel-group intervention study included 53 Japanese college students (mean age, 21.1 ± 0.1; men, n = 40; women, n = 13). They were randomly divided into three groups: control group (n = 14), intervention I group (n = 19), and intervention II group (n = 20). The participants ingested 500 ml/day of commercially available natural water (0 mg of catechin/day) or green tea (intervention I group, 200 mg of catechin/day; intervention II group, 400 mg of catechin/day) for 4 weeks. After 4 weeks of intervention period, no significant changes in body weight, body mass index, pulse wave velocity, and cardio-ankle vascular index were observed in all groups. However, body fat and carbon dioxide output significantly reduced only in the intervention II group. With the level of catechin concentrations increasing, significant decreasing trends were found in body fat changes and respiratory exchange ratio changes. Therefore, these findings suggest that four weeks of regular green tea intake would reduce body fat, but not arterial stiffness, in young adults.