The temple was founded by Hattori Hanzō, the legendary ninja who served Tokugawa Ieyasu. Despite being nicknamed "Hanzō the Ogre" for his spear mastery, he wept and refused to behead his lord's son during seppuku.

Hattori Hanzō's spear, a temple treasure gifted by Tokugawa Ieyasu, was originally 4.2 meters long but is now broken and shortened to 2.58 meters, weighing 7.5 kilograms.

The temple's name comes from Hattori Hanzō's Buddhist name "Saien," which he took to pray for Matsudaira Nobuyasu's soul. He spent his final years as a priest here before dying in 1597.

Behind the main hall stands a large five-ring stone tower memorial built by Hattori Hanzō for Tokugawa Nobuyasu, marked with the Tokugawa three-leaf hollyhock crest and designated a Shinjuku Ward historic site.