Mar Shimun XX Paulos (1885 in Qodshanis, Hakkari, Ottoman Empire – 27 April 1920 in Baquba, Ottoman Empire) served as the 118th Catholicos-Patriarch of the Assyrian Church of the East.

After his brother, Catholicos-Patriarch Mar Shimun XIX Benyamin, was murdered along with 150 of his followers by Simko Shikak (Ismail Agha Shikak), a Kurdish agha, Mar Shimun XX Paulos was elected on 23 March 1918.

On 20 August 1918, for fear of persecution from the Ottoman Turks during their campaign of genocide against Armenians, Assyrians, and other Christians of the Ottoman Empire, the newly elected Catholicos-Patriarch fled with about 60,000 of his people from Urmia in Iran seeking the protection of the British in Iraq.

The survivors were in a camp in Baquba, about 50 kilometers north of Baghdad.

Becoming very sick, Shimun XX spent his short patriarchal days at Mor Mottai monastery belonging to the Syriac Orthodox Church of Antioch.