was the 36th emperor of Japan, according to the traditional order of succession.
The years of his reign lasted from 645 through 654.
Before Kōtoku's ascension to the Chrysanthemum Throne, his personal name (imina) was Karu or Prince Karu.
He was a descendant of Emperor Bidatsu. He was a son of Chinu no ōkimi (Prince Chinu, 茅渟王) by Kibitsuhime no ōkimi (Princess Kibitsuhime, 吉備姫王). Empress Kōgyoku was his elder sister from the same parents. Chinu was a son of Prince Oshisaka hikohito no ōe, whose father was the Emperor Bidatsu. He had at least three consorts including his Empress, Hashihito no Himemiko (Princess Hashihito), the daughter of Emperor Jomei and his sister Empress Kōgyoku. In the 3rd year of Kōgyoku-tennōs reign (皇極天皇三年), the empress abdicated; and the succession (senso) was received by her younger brother. Shortly thereafter, Emperor Kōtoku is said to have acceded to the throne (sokui).
Kōtoku ruled from July 12, 645, until his death in 654. In 645 he ascended to the throne two days after Prince Naka no Ōe (Emperor Tenji) assassinated Soga no Iruka in the court of Kōgyoku. Kōgyoku abdicated in favor of her son and crown prince, Naka no Ōe, but Naka no Ōe insisted Kōtoku should ascend to the throne instead. Kōtoku's contemporary title would not have been tennō, as most historians believe this title was not introduced until the reigns of Emperor Tenmu and Empress Jitō. Rather, it was presumably Sumeramikoto or Amenoshita Shiroshimesu Ōkimi (治天下大王), meaning "the great king who rules all under heaven". Alternatively, Kōtoku might have been referred to as (ヤマト大王/大君) or the "Great King of Yamato". According to the Nihonshoki, he was of gentle personality and was in favor of Buddhism.
In 645 he created a new city in the area called Naniwa, and moved the capital from Yamato Province to this new city (see Nara). The new capital had a sea port and was good for foreign trade and diplomatic activities. In 653 Kōtoku sent an embassy to the court of the Tang dynasty in China, but some of the ships were lost en route.