Empress Kōgyoku (皇極天皇) or Saimei (斉明天皇) (594–661) was the 35th imperial ruler of Japan. From 642 she ruled as Kogyoku, but abdicated after the assassination of Soga no Iruka and gave up the throne to her brother Kotoku in 645. After Kotoku died in 655, she reascended the throne as Saimei, and ruled under that name until her death. She was a great-granddaughter of Emperor Bidatsu. Her birth name was Princess Takara.

She was the wife and empress of the Emperor Jomei. They had three children: Prince Nakanooe (Tenji), Prince Oama (Temmu), and Princess Hashihito.

During her first reign the clan of Soga seized power. Her son Nakanooe planned a coup d'etat and slew Soga no Iruka at the court in front of her throne. The empress was very shocked with this incidence and abdicated the throne.

After Emperor Kotoku died, though Nakanooe was the crown prince, he had his mother reascend the throne, and remained as the crown prince under his mother. He, and not his mother, however, led the politics of Japan. In the fifth year of her second reign, Baekje in Korea was destroyed in 660. Japan assisted Baekje loyals to the attempt of retrieving former Baekje territory. Early in 661, Saimei started from the capital in Yamato province in Honshu with both an army and a navy and crossed the Seto Inland Sea from east to west. The empress stayed in Ishiyu Temporary Palace in Iyo province, today Dogo spa. In May she arrived at Asakura Palace in the north part of Chikushi province in Kyushu, today a part of Fukuoka prefecture. The allied army of Japan and Baekje was prepared the war against Silla but on July 24 (Japanese calendar), 661 she died in the Asakura Palace before the army departed to Korea. In October her body was brought from Kyushu by sea to Port Naniwa-zu (today Osaka city). Her funeral ceremony was held in early November.

After her death, her son Nakanooe ascended to the throne in 663, after the battle against Silla and the Sui Dynasty.