Mar Benyamin Shimon XXI (1887 - March, 1918) was a Catholicos Patriarch of the Assyrian Church of the East. He was born in 1887 in the village of Quchanis in the district of Hakkari, Turkey. He was consecrated a Metropolitan on March 1, 1903 by the late Mar Rowil Catholicos Patriarch who passed away on March 16, 1903. He, consequently, succeeded his predecessor at the age of eighteen, and thus, occupied the patriarchal See of Seleucia-Ctesiphon at Quchanis for 15 years. He was assassinated in March, 1918 by a Kurdish paramilitary group under a truce flag.