Shinzen (804 - October 21, 891) was a Shingon Sect Buddhist monk who lived during the first part of the Heian period. He was born into the Saeki clan. He came from Tado County, Sanuki Province. He is also known as Naka-no-In Sojo and Nochino Sojo. It is said that he was the nephew of Kukai.
After entering the Buddhist priesthood, he resided at Daian-ji Temple in Nara where he studied Shingon Esoteric Buddhism under Shinga and put great effort into the administration of Mt. Koya following Kukai's death. In the year 836, he attempted to travel to Tang Dynasty China with Shinzei but failed. He ascended to the rank of Gon Risshi (generally in Shingon sect, fifteenth-ranking Buddhist priest, literally, "supernumerary master of discipline") in the year 874 before being promoted to Sojo (high-ranking Buddhist priest). He was appointed chief abbot of To-ji Temple in 884, a post that he held until his death in 891. His removing of the 'Sanjujo Sasshi' (30 volumes of esoteric Buddhist scripture hand-copied by priest Kukai when he was staying in China) lent to him by Shinga from Mt. Koya during this period is said to have been the origin of a dispute between To-ji Temple and Mt. Koya.