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. Abandoned in 1497 due to Muslim persecutions, the See of Bacău was re-established in 1611, and had a succession of twenty prelates until 1789, when it was suppressed. 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 Apostolic Vicariate of Moldavia was promoted on 27 June 1884 by Pope Leo XIII to Diocese of Iaşi (Jászvásár in Hungarian) covering Romanian Moldavia, with Iaşi as residence.
As of 2014[update], its pastorally served 232,132 Catholics (5.3% of 4,359,762 total) on 46,378 km2 in 149 parishes, 420 priests (297 diocesan, 123 religious) with 704 lay religious (282 brothers, 422 sisters) and 125 seminarians.
5.2% of the inhabitants are Catholic, with concentrations around Bacău and Roman. Its adherents are predominantly ethnic Romanians, with small Csángós (old Hungarian dialect) and Polish communities.
Nicolae Iosif Camilli (1884.06.27 – 1894.05.06 see below), previously Titular Bishop of Mosynopolis (1881.09.16 – 1884.06.27); later Titular Bishop of Gadaræ (1896.02.25 – 1901.03.27), Titular Archbishop of Constantia antea Tomi (1901.03.27 – 1904.08.30)
Dominique Jacquet (1895.01.08 – 1904.02.25), also Metropolitan Archbishop of București (Bucharest, Romania) (1895–1895); later
Nicolae Iosif Camilli (see above 1904.08.30 – 1916.01.17), now styled Archbishop-Bishop of Iaşi (Romania) (1904.08.30 – 1916.01.17)
Alexandru Theodor Cisar (1920.04.22 – 1924.12.12), later Metropolitan Archbishop of București (Romania) (1924.12.12 – death 1954.01.07)