| Type: | Catholicos-Patriarch of the Assyrian Church of the East |
| Honorific-Prefix: | Mar |
| Shimun XVIII Rubil | |
| His Holiness | |
| Church: | Assyrian Church of the East |
| Diocese: | Patriarchal Diocese of Qodshanis |
| See: | Holy Apostolic See of Seleucia-Ctesiphon |
| Enthroned: | 1861 |
| Ended: | 1903 |
| Predecessor: | Mar Shimun XVII Abraham |
| Successor: | Mar Shimun XIX Benyamin |
| Rank: | Catholicos-Patriarch |
| Birth Place: | Qodshanis, Hakkari |
| Death Date: | 1903 |
| Death Place: | Qodshanis, Hakkari |
| Nationality: | Assyrian (Ottoman) |
| Religion: | Christian, Assyrian Church of the East |
| Residence: | Qodshanis, Hakkari |
Mar Shimun XVIII Rubil (also Simon XVIII Rouel or Rowil) served as the Catholicos-Patriarch of the Assyrian Church of the East from 1861 to 1903, succeeding his uncle Shimun XVII Abraham.
He led the church from Qodshanis, in southeastern Turkey. In 1869, he received an invitation from the Vatican to attend the First Vatican Council as an observer, but he did not accept the invitation, and he also rejected other initiatives for the union with the Catholic Church. The Catholicos-Patriarch died on March 16, 1903, and was succeeded by Shimun XIX Benyamin.