Matsudaira Kiyoyasu (松平 清康?, September 28, 1511 – 1536) was the 7th lord over the Matsudaira clan during the Sengoku period (16th century) of Japan. Kiyoyasu was the grandfather of the exceedingly famous unifier of Japan, Tokugawa Ieyasu. Kiyoyasu soon gained control of the whole of the northern region of Mikawa province after the Saigo clan had surrendered after four generations of conflict between the two clans. The Okazaki castle was also built as a monument towards the Matsudaira's power. However, certain hatred began growing within a certain retainer of Kiyoyasu's, a retainer by the name of Abe Masatoyo. During the year of 1535, this certain retainer had somehow sneaked into Kiyoyasu's secret chambers and slew him with his Muramasa blade. After the revolt had been subdued at the battle of Idano, and peace returned to the Matsudaira, Matsudaira Hirotada, father of Ieyasu, would succeed to the throne of power within the Matsudaira clan.
Another version of Matsudaira Kiyoyasu's death has been told by the author A. L. Sadler:
"Kiyoyasu, the son of Nobutada, was a fine soldier, and his friendship was solicited both by Takeda Nobutora, father of the great Shingen, and also by Oda Nobuo mitsu, uncle of the more famous Nobunaga. Oda later made secret overtures to the effect that if Kiyoyasu attacked his province he would be on his side, his intention being to oust his elder brother Nobuhide, the head of the clan. So Kiyoyasu set out against this province. But his wicked uncle Nobusada, seeing an opportunity, sent to Nobuhide to say that he was about to take the Castle of Anjo, the headquarters of Kiyoyasu, from which he had set out. When Kiyoyasu heard of this he was naturally very troubled at the possibility of his base being taken behind his back, and he was rendered more so by another rumour started by someone that his most faithful retainer Abe Sadayoshi was also in league with his uncle. Abe Sadayoshi was very indignant when he heard this slander, and called his son Yashichi[disambiguation needed], telling him that it was false, and would be proved so if proper examination was made. But if this was not done, and he was put to death on suspicion, he impressed on him the need of his continuing to serve their lord faithfully as if nothing had happened. Just after this Sadayoshi's horse began to be restless and kick out, and there was some confusion, and Kiyoyasu came out and gave orders to catch it and tie it up. Hearing the noise, Yashichi at once concluded that his father was being arrested and was in danger, and without more Ado rushed out on the spur of the moment without any reflection and cut Kiyoyasu down. He was at once killed himself, but that did not save Kiyoyasu, who was only twenty-five. But he was not without an heir, his son Hirotada being ten years old. The army of Kiyoyasu had to retire immediately he was killed, and it was Sadayoshi who took charge of his son, for the charge of treason seems to have been quickly shown to be false, and he was trusted as before."
- The Samurai Sourcebook
- A. L. Sadler, The Maker of Modern Japan: The Life of Shogun Tokugawa Ieyasu, Tuttle 1937, pp. 38–39