Endō Tanenao served as a bushō during the Azuchi-Momoyama to early Edo periods.
Tanenao was born as the son of Endō Tanemoto of the Kigoshi-Endō clan.
On 11/23 of Bunroku 2 (1593), Tanemoto died of illness at the Kokubun Temple in Nagato Province while returning from service in the Bunroku Campaign on the Korean Peninsula. He was forty-nine years old. Tanenao inherited his landholdings, becoming the lord of Inuji Castle in Mino with a fief of 5,500 koku. In 1594, Tanenao joined Endō Yoshitaka (his father-in-law) to harvest lumber in eastern Mino for the construction of Fushimi Castle for Toyotomi Hideyoshi. He further participated in the building project so was provided a residence in Ōkamidani to the north of Fushimi Castle. On 12/7 of Keichō 2 (1597), he made donations to the Hakusan-Gongen, a goddess of Shinto-Buddhist syncretism.
In 1600, at the Battle of Sekigahara, Tanenao, together with Yoshitaka, initially pledged their allegiance to Sakakibara Yasumasa, a senior retainer of Tokugawa Ieyasu and joined the Eastern Army. Without notice, he switched sides to the Western Army and holed-up in Uegane Castle. On 8/1, a letter of encouragement, thirty arquebuses and gunpowder arrived from Oda Hidenobu, the lord of Gifu Castle. In one of the preliminary battles prior to the main Battle of Sekigahara, Tanenao lost at the Siege of Hachiman Castle. In the midst of dimming prospects, on 9/5, he was attacked by Yoshitaka at Uegane Castle and surrendered. After the war, Yoshitaka’s son-in-law, Kanamori Arishige, requested Tokugawa Ieyasu to pardon Tanenao, but Ieyasu did not permit Tanenao’s defiance of Yoshitaka and removed him from his position. Following his dismissal, Tanenao separated from his wife and became a rōnin, or wandering samurai, moving his residence to Kyōto and adopting the monk’s name of Gensai.
There are several theories regarding the year of his death which is surmised to have been in 1604.