Breast cancer is one of the major cancers in women, and dietary intake must be controlled to prevent it. Selenium (Se), especially Se compound in vegetables, is thought to be a promising chemopreventive dietary ingredient for preventing breast cancer. In this study, we developed Se-enriched Japanese radish sprout using a special Se-additional fertilizer, and identified the Se chemical forms. The newly developed Se-enriched sprout is produced within a week by the tank forming method, and the major chemical form was identified as Se-methylselenocysteine (MeSeCys) (80%). Then, the chemopreventive effects of the Se-enriched sprout were investigated using Sprague-Dawley female rats with mammary cancer, induced by a single oral dose of 10 mg or 14 mg of 7, 12-dimethylbenz[a]anthracene (DMBA). Mammary tumors were found in 11, 16 and 2 rats treated with DMBA and thereafter fed the basal (n = 34), sprout-added basal (n = 30) and Se-enriched sprout-added test diets (n = 30), respectively. The incidence of mammary tumors was significantly lower in the Se-enriched sprout-added test diet group (7%) than in the basal diet group (32%) or sprout-added basal diet group (53%). In contrast, no significant difference was detected in the numbers and incidence of the tumor between the basal diet group and Se-enriched sprout-added test diet group before DMBA-dosing. These results suggest that the diet supplement of Se-enriched sprout after DMBA-dosing provides a significant chemoprevention against chemical-induced mammary cancer. Thus, Se-enriched sprout may be a useful dietary ingredient for preventing breast cancer.