The Diocese of Iași (or Latin: Dioecesis Iassiensis, Romanian: Episcopia Romano-Catolică de Iași, Hungarian: Jászvásári Egyházmegye, Polish: Diecezja Jassy) is a Latin Church ecclesiastical territory or diocese of the Catholic Church in Romanian Western Moldavia.

It was established in 1818 as the Apostolic Vicariate of Moldavia (a pre-diocesan missionary jurisdiction; exempt, i.e. directly subject to the Holy See, not part of an ecclesiastical province, but entitled to a titular bishop), on territory split off from the suppressed Diocese of Bacău, which had originally been set up at Siret by Pope Urban V in 1370, due to work done by Franciscans and Dominicans; its seat was transferred to Bacău at the beginning of the 15th century.

The Catholics of Moldavia were then placed under the spiritual direction of Apostolic prefects, generally chosen from the Conventuals in charge of the mission.

The diocese covers the Romanian region of Moldavia—the counties of Suceava, Botoșani, Neamț, Iași, Bacău, Vaslui, Vrancea and Galați.

Its adherents are predominantly ethnic Romanians, with small Csángós (old Hungarian dialect) and Polish communities.