Matsudaira Sadaaki (松平 定敬, January 18, 1847 – July 12, 1908) was a Japanese daimyō of the Bakumatsu period, who was the last ruler of the Kuwana Domain.

Sadaaki was the adopted heir of Matsudaira Sadamichi, the descendant of Sadatsuna, the third son of Hisamatsu Sadakatsu (1569–1623), who was Tokugawa Ieyasu's brother.

[2] Matsudaira Sadaaki was appointed Kyoto Shoshidai in the period spanning May 16, 1864, through January 3, 1868 and was thus the last person to hold this post.

In 1864, Sadaaki deployed Kuwana troops during then Kinmon incident and as part of the shogunate's effort to subdue the Tengu-tō uprising.

During these years Sadaaki was famous as an avid horseman, and received an imported Arabian horse as a gift from the Shogunate.

Yoshinobu, Katamori and Sadaaki withdrew to Osaka Castle and a large military force from Satsuma and Chōshū occupied Kyoto, demanding that the Tokugawa and its cadet houses (including the Matsudaira) be stripped of their title and domains.

Unable to return to Kuwana, as the Satchō Alliance army was in the way, he travelled instead to Kashiwazaki in Echigo Province, as this was an exclave of Kawana Domain, using a Prussian steamship "Costa Rica" from Osaka.

[5] After the fall of Yonezawa and the defeat of the Ōuetsu Reppan Dōmei, Sadaaki embarked on Enomoto Takeaki's warships at Matsushima Bay, and went on to the Ezo Republic.

In 1873, together with his son Matsudaira Sadanori, he enrolled at a school founded by American missionary Samuel Robbins Brown to study the English language.