was . He was the eldest son of Prince Shirakabe (later known as Emperor Konin), and was born prior to Shirakabe's ascension to the throne. According to the , Yamabe's mother, Yamato no Niigasa (later called Takano no Niigasa), was a 10th generation descendant of Muryeong of Baekje (462-523).

After his father became emperor at the age of 61, Kammu's half-brother, Prince Osabe was appointed to the rank of crown prince.