Sainen-ji (西念寺) is a Buddhist temple belonging to the Jà Âdo-shà « sect, located in Shinjuku Ward, Tokyo, Japan. It is known as the temple founded by Hattori Hanzà Â, a famous samurai of the Sengoku Period who served Tokugawa Ieyasu. The temple is the family temple of the Hattori clan, and contains the graves of Hanzà  and other members of the Hattori clan. In addition, there is a memorial tower that is said to have been built by Hanzà  for Tokugawa Ieyasu's eldest son, Nobuyasu, whom Hanzà  served as guardian.
Sainen-ji Temple was founded in 1594, shortly after the Tokugawa clan took possession of Edo, when Hattori Hanzà  founded its predecessor, An'yoin, in Shimizudani, Kojimachi, Edo (near modern-day Shimizudani Park in Kioicho, Chiyoda Ward). Hanzà  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. After his death in 1597, he was posthumously given the Buddhist name Senshoinden An'yo Saien Daizenjomon and was buried in the same hermitage.
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. The current mountain name and temple name of Sainen-ji are derived from his posthumous name.