Sakai Tadanao served as a bushō during the Sengoku period. He was a retainer of the Matsudaira (Tokugawa) clan and served as the lord of Ueno Castle in Mikawa Province.
Tadanao was born as the son of Sakai Yasutada and may have been the uncle of Sakai Tadatsugu but the relationship is uncertain. From the era of Matsudaira Hirotada, he served as a senior retainer of the Matsudaira clan. After the demise of Hirotada, Tadanao served Matsudaira Motoyasu (later known as Tokugawa Ieyasu) and, as a senior retainer, his signature appears on documents of the Matsudaira family. Tadanao had so much capability and spirit of independence that Imagawa Yoshimoto treated him separately from Motoyasu.
In 1563, Tadanao rebelled against Motoyasu and, in the sixth month, holed-up in Ueno Castle against a deployment by Motoyasu. The common view is that he joined the Mikawa Ikkō-ikki, but his rebellion occurred earlier than the Ikkō-ikki and there are no traces of communications between Tadanao and the ikki forces. In the twelfth month, as the Ikkō-ikki approached Okazaki Castle in Mikawa, Tadanao continued to hold-out in the castle without traces of a counterattack. When, in the spring of the following year, Motoyasu and the Ikkō-ikki settled, Tadanao continued to resist. According to chronicles of the Matsudaira, the resistance collapsed on 9/6 of Eiroku 7 (1564). Tadanao fled from Ueno Castle to Suruga Province and his whereabouts thereafter are unknown.