| Honorific-Prefix: | His All Holiness |
| Constantius I | |
| Patriarch Of: | Ecumenical Patriarch of Constantinople |
| Enthroned: | 6 July 1830 |
| Ended: | 18 August 1834 |
| Diocese: | Constantinople |
| See: | Ecumenical Patriarchate |
| Church: | Church of Constantinople |
| Predecessor: | Agathangelus |
| Successor: | Constantius II |
| Birth Date: | 1777 |
| Birth Place: | Istanbul |
| Death Date: | January 5, 1859 |
| Death Place: | Burgazada |
| Religion: | Eastern Orthodox Church |
| Occupation: | Ecumenical Patriarch |
Constantius I (Greek, Modern (1453-);: Κωνστάντιος; 1777 – 5 January 1859) was Ecumenical Patriarch during the period 1830–1834.
He was born in 1777 in Constantinople.[1] He studied in the Patriarchal School, in Iași and in Kiev. In 1805, he was elected Archbishop of Sinai, a position he held until he was elected Ecumenical Patriarch in 1830. He resigned in 1834 and devoted his life to studying and writing. He died on 5 January 1859.[2]