Mar Eshai Shimun XXIII (26 February 1908 – 6 November 1975), sometimes known as Mar Shimun XXIII Ishaya, Mar Shimun Ishai, or Simon Jesse, was Catholicos Patriarch of the Assyrian Church of the East from 1920, when he was a youth, until his murder on 6 November 1975.
(The difference in regnal numbers depends on which members of the Shimun family one counts as Patriarchs; Mar Eshai chose to use the regnal number XXIII.)