The effect of cross-border diversification on bank performance is part of the broader debate on how multinational banking and financial integration affect the global financial economy. Previous studies that examined this relationship present mixed results - namely that cross-border diversification improves bank performance but also increases bank risks that could lead to systemic failure. Even so, this line of debate has not been examined in the case of Japanese banks conducting international operations. The present study questions whether cross-border diversification improves the performance of Japanese banks and to what extent each cross-border expansion activity affects bank performance. The latter was largely ignored in previous studies. Our results show that cross-border diversification improves cost efficiency but decreases/harms the profit efficiency of the banks analyzed. In addition, we find that the expansion of foreign assets and foreign branch operations present funding risks and operational inefficiency. We offer two important recommendations. First, as a major player in international lending, the current expansion activities of Japanese banks require close monitoring and supervision to prevent systemic risk resulting from aggressive and risky overseas expansion activities. Second, the current expansion strategies of Japanese banks, especially the expansion of overseas assets and branch operations (retail banking), should be re-examined.