Date | Country | Change |
| 301 |  Italy | Diocese of Voghenza was established |
| 306 |  Portugal | Diocese of Ossonoba was established |
| 308 | Italy | Diocese of Sala Consilina was established |
| 310 |  Spain | Diocese of León was suppressed (to Diocese of Astorga) |
| 312 |  Algeria | Diocese of Cirta was renamed as Diocese of Constantine |
| 312 | Italy | Diocese of Grumentum was established |
| 313 | Italy | Diocese of Atella was established |
| 313 | Italy | Diocese of Liternium was established |
| 314 | Portugal | Diocese of Betecas was established |
| 323 | Italy | Diocese of Acqui was established |
| 323 | Italy | Diocese of Aosta was established |
| 325 |  Lebanon | Patriarchal See of Antioch (Maronite Rite) was established |
| 325 | Lebanon | Patriarchal See of Antioch (Syriac Rite) was established |
| 325 |  Palestine,  Cyprus,  Israel,  Jordan | Diocese of Aelia Capitolina was renamed as Diocese of Jerusalem |
| 325 | Spain | Diocese of Cartagena was promoted as Metropolitan Archdiocese of Cartagena |
| 325 |  Syria | Metropolitan Archdiocese of Damascus was established |
| 325 |  Turkiye | Diocese of Antioch was promoted as Patriarchal See of Antioch |
| 325 | Turkiye | Diocese of Myra was promoted as Metropolitan Archdiocese of Myra |
| 326 | Italy | Diocese of Albano was established (became suffragan of Roma) |
| 330 |  France | Diocese of Toul was established |
| 330 | Turkiye | Diocese of Byzantium was promoted as Metropolitan Archdiocese of Constantinople |
| 330 | Turkiye | Diocese of Iconium was promoted as Metropolitan Archdiocese of Iconium |
| 340 | France | Diocese of Verdun was established |
| 340 |  Germany | Diocese of Mainz was established |
| 340 | Germany | Diocese of Speyer was established |
| 340 | Italy | Diocese of Bari was established |
| 340 | Italy | Diocese of Canosa was established |
| 344 |  Belgium | Diocese of Tongeren was established (from Diocese of Köln) |
| 346 |  Switzerland | Diocese of Basel was established |
| 350 |  Albania | Diocese of Shkodrë was established |
| 350 | Algeria | Diocese of Midila was established |
| 350 | France | Diocese of Bayeux was established |
| 350 | France | Diocese of Nice was established (from Diocese of Marseille) |
| 350 |  Greece | Diocese of Patrasso was established |
| 350 | Italy | Diocese of Albenga was established |
| 350 | Italy | Diocese of Brescia was established |
| 350 | Italy | Diocese of Imola was established |
| 350 | Italy | Diocese of Narni was established |
| 350 | Italy | Diocese of Pavia was established |
| 350 | Italy | Diocese of Pitinum Merges was established (without ecclesiastical province) |
| 350 | Italy | Diocese of Tivoli was established (without ecclesiastical province) |
| 350 | Portugal | Diocese of Aquae Flaviae was established |
| 350 | Portugal | Diocese of Olisipo was established |
| 350 | Portugal | Diocese of Salácia was suppressed |
| 350 | Spain | Diocese of Cástulo was established (from Diocese of Tucci) |
| 350 | Spain | Diocese of Emerita Augusta was promoted as Metropolitan Archdiocese of Emerita Augusta (with suffragan see Ossonoba) |
| 350 | Spain | Diocese of Mentesa was established |
| 354 | France | Diocese of Embrun was established |
| 355 | Italy | Diocese of Aquileia was promoted as Metropolitan Archdiocese of Aquileia (with suffragan sees Pavia, Poreč, Trento and Verona) |
| 370 | France | Diocese of Saint-Paul-Trois-Châteaux was established |
| 372 | France | Diocese of Angers was established |
| 372 | Italy | Diocese of Brescello was established (from the suppressed Diocese of Parma) |
| 372 | Italy | Diocese of Parma was suppressed (to establish Diocese of Brescello) |
| 374 | France | Diocese of Vence was established |
| 374 | Italy | Diocese of Lodi was established (became suffragan of Milano) |
| 374 | Italy | Diocese of Milano (Ambrosian Rite) was promoted as Metropolitan Archdiocese of Milano (with suffragan sees Acqui, Albenga, Asti, Bologna, Brescia, Faenza, Genova, Lodi, Modena, Tortona, Vercelli and Voghenza) |
| 375 | Italy | Diocese of Altino was established |
| 380 | Belgium | Diocese of Tongeren was renamed as Diocese of Tongeren en Maastricht |
| 380 |  Croatia | Diocese of Zadar was established |
| 380 | France | Diocese of Grenoble was established (from Diocese of Vienne) |
| 380 | Italy | Diocese of Concordia was established (became suffragan of Aquileia) |
| 380 | Italy | Diocese of Zuglio was established |
| 380 | Portugal | Diocese of Braga was established |
| 380 |  Slovenia | Diocese of Emona was established |
| 380 | Spain | Diocese of Ávila was established (became suffragan of Emerita Augusta) |
| 380 | Switzerland | Diocese of Octodurum was established |
| 380 | Syria | Diocese of Callinicum was established |
| 386.11.01 | Italy | Diocese of Como was established (from Metropolitan Archdiocese of Milano and became suffragan of Milano) |
| 390 | France | Diocese of Nîmes was established |
| 395 |  Serbia | Diocese of Srijem was promoted as Metropolitan Archdiocese of Srijem |
| 397 | Italy | Diocese of Novara was established (became suffragan of Milano) |
| 400 | Albania | Diocese of Amantia was suppressed |
| 400 | Albania | Diocese of Apollonia was established |
| 400 | Albania | Diocese of Drivasto was established |
| 400 | Albania | Metropolitan Archdiocese of Durrës was established |
| 400 | Albania | Diocese of Hadrianopolis in Epiro was established |
| 400 | Albania | Diocese of Lestrona was established |
| 400 | Albania | Diocese of Onchesmus was established |
| 400 | Albania | Diocese of Phœnice was established |
| 400 | Albania | Diocese of Scampa was established |
| 400 | Algeria | Diocese of Gegi was established |
| 400 | Algeria | Diocese of Icosium was established |
| 400 | Algeria | Diocese of Jomnium was established |
| 400 | Algeria | Diocese of Rusguniæ was established |
| 400 |  Austria | Diocese of Juvavo was established |
| 400 | Croatia | Diocese of Senj was established |
| 400 |  Egypt | Diocese of Nitria was suppressed |
| 400 | France | Diocese of Agen was established (became suffragan of Bordeaux) |
| 400 | France | Diocese of Aire was established (became suffragan of Eauze) |
| 400 | France | Diocese of Antibes was established |
| 400 | France | Diocese of Auch was established (became suffragan of Eauze) |
| 400 | France | Diocese of Avignon was established |
| 400 | France | Diocese of Bazas was established (became suffragan of Eauze) |
| 400 | France | Diocese of Besançon was promoted as Metropolitan Archdiocese of Besançon (with suffragan see Basel) |
| 400 | France | Metropolitan Archdiocese of Bourges was established (with suffragan sees Albi, Auvergne, Cahors, Le Puy-en-Velay and Limoges) |
| 400 | France | Diocese of Béziers was established |
| 400 | France | Diocese of Carpentras was established |
| 400 | France | Diocese of Cavaillon was established |
| 400 | France | Diocese of Châlons-sur-Marne was established (became suffragan of Reims) |
| 400 | France | Diocese of Digne was established |
| 400 | France | Diocese of Fréjus was established |
| 400 | France | Diocese of Gap was established |
| 400 | France | Diocese of Nantes was established |
| 400 | France | Diocese of Reims was promoted as Metropolitan Archdiocese of Reims (with suffragan sees Amiens, Beauvais, Châlons-sur-Marne and Soissons) |
| 400 | France | Diocese of Riez was established |
| 400 | France | Diocese of Saint-Bertrand-de-Comminges was established (became suffragan of Eauze) |
| 400 | France | Diocese of Saintes was established (became suffragan of Bordeaux) |
| 400 | France | Diocese of Sisteron was established |
| 400 | France | Diocese of Toulon was established |
| 400 | France | Diocese of Valence was established |
| 400 | France | Diocese of Vernay was established |
| 400 | France | Diocese of Alba Helvianorum was renamed as Diocese of Viviers |
| 400 | Greece | Diocese of Argos was established |
| 400 | Greece | Diocese of Carystus was established (became suffragan of Corinthus) |
| 400 | Greece | Diocese of Coronea was established |
| 400 | Greece | Diocese of Rhodes was promoted as Metropolitan Archdiocese of Rhodes |
| 400 | Greece | Diocese of Thespia was established |
| 400 |  Iraq | Diocese of Seleucia–Ctesifonte (Chaldean Rite) was promoted as Metropolitan Archdiocese of Seleucia–Ctesifonte |
| 400 |  Ireland | Diocese of Emly was established |
| 400 | Ireland | Abbacy nullius of Trim was established |
| 400 | Italy | Diocese of Acerenza was established |
| 400 | Italy | Diocese of Alba was established (became suffragan of Milano) |
| 400 | Italy | Diocese of Anzio was established |
| 400 | Italy | Diocese of Ascoli Piceno was established |
| 400 | Italy | Diocese of Bergamo was established (became suffragan of Milano) |
| 400 | Italy | Diocese of Bettona was established |
| 400 | Italy | Diocese of Bomarzo was established |
| 400 | Italy | Diocese of Brindisi was established |
| 400 | Italy | Diocese of Castellammare di Stabia was established |
| 400 | Italy | Diocese of Caudium was established |
| 400 | Italy | Diocese of Celano was established |
| 400 | Italy | Diocese of Cingoli was established |
| 400 | Italy | Diocese of Cornus was established |
| 400 | Italy | Diocese of Egnazia Appula was established |
| 400 | Italy | Diocese of Ferento was established |
| 400 | Italy | Diocese of Fiesole was established |
| 400 | Italy | Diocese of Gubbio was established (without ecclesiastical province) |
| 400 | Italy | Diocese of Lilibeo was established |
| 400 | Italy | Diocese of Lucera was established |
| 400 | Italy | Diocese of Matelica was established |
| 400 | Italy | Diocese of Nocera Umbra was established |
| 400 | Italy | Diocese of Nomento was established |
| 400 | Italy | Diocese of Osimo was established (without ecclesiastical province) |
| 400 | Italy | Diocese of Otranto was established |
| 400 | Italy | Diocese of Otricoli was established |
| 400 | Italy | Diocese of Pesto was established |
| 400 | Italy | Diocese of Piacenza was established (became suffragan of Milano) |
| 400 | Italy | Diocese of Pitinum was suppressed (to Diocese of Amiterno) |
| 400 | Italy | Diocese of Plestia was established |
| 400 | Italy | Diocese of Pollentia was established |
| 400 | Italy | Diocese of Potenza Picena was established |
| 400 | Italy | Diocese of Sarsina was established |
| 400 | Italy | Diocese of Siponto was established |
| 400 | Italy | Diocese of Squillace was established (without ecclesiastical province) |
| 400 | Italy | Diocese of Termini Imerese was established |
| 400 | Italy | Diocese of Tharros was established |
| 400 | Italy | Diocese of Tolentino was established |
| 400 | Italy | Diocese of Torino was established (became suffragan of Milano) |
| 400 | Italy | Diocese of Treia was established |
| 400 | Italy | Diocese of Treviso was established (became suffragan of Aquileia) |
| 400 | Italy | Diocese of Urbisaglia was established |
| 400 | Italy | Diocese of Volterra was established |
| 400 |  Montenegro | Diocese of Doclea was established |
| 400 |  North Macedonia | Metropolitan Archdiocese of Dardania was established |
| 400 | Palestine | Diocese of Maiumas Gazæ was established |
| 400 | Portugal | Diocese of Betecas was suppressed |
| 400 | Portugal | Diocese of Coinimbriga was established (from Diocese of Olisipo and became suffragan of Emerita Augusta) |
| 400 | Serbia | Diocese of Horreomargum was suppressed |
| 400 | Serbia | Metropolitan Archdiocese of Viminacium was suppressed |
| 400 | Slovenia | Diocese of Ptuj was suppressed |
| 400 | Spain | Diocese of Barcelona was established |
| 400 | Spain | Diocese of Complutum was established (from Diocese of Toledo) |
| 400 | Spain | Diocese of Cástulo was suppressed (to Diocese of Tucci) |
| 400 | Spain | Diocese of Gerona was established |
| 400 | Spain | Diocese of Menorca was established |
| 400 | Spain | Diocese of Sevilla was promoted as Metropolitan Archdiocese of Sevilla (with suffragan see Córdoba) |
| 400 | Spain | Diocese of Tortosa was established |
| 400 | Switzerland | Diocese of Genève was established |
| 400 | Syria | Diocese of Aleppo (Greek-Melkite Rite) was established |
| 400 |  Tunisia | Diocese of Menefessi was established |
| 400 | Tunisia | Diocese of Ruspæ was established |
| 400 | Tunisia | Diocese of Sufes was established |
| 400 | Tunisia | Diocese of Theudalis was established |
| 400 | Tunisia | Diocese of Tunes was established |
| 400 | Tunisia | Diocese of Vita was established |
| 400 | Turkiye | Diocese of Achyraus was established |
| 400 | Turkiye | Archdiocese of Nisibis was suppressed |
| 400 | Turkiye | Diocese of Polybotus was established |
| 400 | Turkiye | Diocese of Sebastopolis in Thracia was suppressed |
