Emperor Kojong died suddenly in January 1919. Immediately, rumors circulated widely that he had been poisoned by the Japanese, and wall posters to that effect appeared throughout Seoul. The people who gathered to mourn Kojong’s death engaged in peaceful demonstrations, which served as the impetus of the March First Movement in Korea. The Japanese authorities never investigated the cause of Kojong’s sudden death, which has remained a mystery to this date. There is strong evidence that he was poisoned. Moreover, Princess Masako, a daughter-in-law of Kojong, testifies in her autobiography that Japanese government officials gave a secret instruction to poison him. Consistent with her testimony, there is an allegation in Kuratomi Yūzaburō’s diary that Terauchi Masatake and Hasegawa Yoshimichi may have been behind the crime. Our review of the evidence strongly supports the allegation that the highest Japanese officials were behind the poisoning of Emperor Kojong.