The relationships between social dominance orientation (SDO) and political attitudes and prejudice have been investigated mainly in Western societies, with little known about them in other societies, especially in Japan. In this study, we examined the external validity of the SDO scale and the relationship between SDO and discriminatory attitudes toward foreigners using samples of Japanese undergraduates and adults from the general population. Across four studies, the results confirmed the reliability of the Japanese SDO scale and found that SDO positively correlated with political conservatism, such as taking a firm stance on the “Takeshima” Island (“Dokdo” in Korean) or “Senkaku” Islands (“Diaoyu Islands” in Chinese and “Diaoyutai Islands” in Taiwanese) problems, negative and avoidant attitudes towards foreigners, and discriminatory attitudes toward Korean residents in Japan. We also found that some gender differences of political attitudes and prejudice were statistically mediated by the gender difference of SDO. These results suggest that SDO can be one of valid explanations to understand the individual differences in Japanese political and discriminatory attitudes.