Sainen-ji (西念寺) is a Buddhist temple belonging to the Jōdo-shū sect, located in Shinjuku Ward, Tokyo, Japan.

It is known as the temple founded by Hattori Hanzō, a famous samurai of the Sengoku Period who served Tokugawa Ieyasu.

Hanzō took the Buddhist name Saien to commemorate the soul of Matsudaira Nobuyasu, who had died an untimely death.

He built a memorial tower at An'yoin and spent the rest of his life praying for Nobuyasu's soul.

[1] Later, due to the expansion of Edo Castle in the surrounding area, An'yo-in was moved to its current location in around 1634 and newly built as a temple.