Date | Country | Change |
| 1818 |  Italy | Diocese of Bobbio lost territory to Diocese of Tortona |
| 1818 |  Réunion | Apostolic Prefecture of Islands of the Indian Ocean was renamed as Apostolic Prefecture of Bourbon |
| 1818 |  Romania | Diocese of Bacău was suppressed (to establish Apostolic Vicariate of Moldavia) |
| 1818 | Romania | Apostolic Vicariate of Moldavia was established (from the suppressed Diocese of Bacău) |
| 1818 |  Russia | Diocese of Smoleńsk was suppressed (to Metropolitan Archdiocese of Mohilev) |
| 1818.01.31 | Italy | Diocese of Gubbio lost territory to Diocese of Cagli |
| 1818.01.31 | Italy | Diocese of Nocera Umbra lost territory to Diocese of Cagli |
| 1818.01.31 | Italy | Abbacy nullius of Nonantola lost territory to Diocese of Cagli |
| 1818.02.23 |  Trinidad and Tobago | Apostolic Vicariate of Trinidad was established (from Diocese of Santo Tomás de Guayana) |
| 1818.03.18 |  Turkiye | Apostolic Vicariate of Smirne was promoted as Metropolitan Archdiocese of Izmir |
| 1818.04.01 |  France | Diocese of Metz lost territory to Diocese of Speyer |
| 1818.04.01 |  Germany | Diocese of Augsburg lost territory to Diocese of Brixen |
| 1818.04.01 | Germany | Diocese of Bamberg was promoted as Metropolitan Archdiocese of Bamberg (with suffragan sees Eichstätt, Speyer and Würzburg and gained territory from Diocese of Würzburg) |
| 1818.04.01 | Germany | Diocese of Chiemsee was suppressed (to Metropolitan Archdiocese of Salzburg and to establish Metropolitan Archdiocese of München und Freising) |
| 1818.04.01 | Germany | Diocese of Konstanz lost territory to Diocese of Brixen |
| 1818.04.01 | Germany | Diocese of Mainz lost territory to Diocese of Konstanz, Diocese of Speyer and Diocese of Würzburg |
| 1818.04.01 | Germany | Metropolitan Archdiocese of München und Freising was restored (from Metropolitan Archdiocese of Salzburg, from the suppressed Diocese of Chiemsee and with suffragan sees Augsburg and Regensburg) |
| 1818.04.01 | Germany | Diocese of Speyer lost territory to Diocese of Konstanz |
| 1818.04.01 | Germany | Diocese of Würzburg lost territory to Metropolitan Archdiocese of Bamberg and Diocese of Konstanz |
| 1818.05.01 | Italy | Diocese of Adria exchanged territory with Metropolitan Archdiocese of Ferrara |
| 1818.05.01 | Italy | Diocese of Belluno lost territory to Diocese of Treviso |
| 1818.05.01 | Italy | Diocese of Caorle was suppressed (to Patriarchal See of Venezia) |
| 1818.05.01 | Italy | Diocese of Concordia exchanged territory with Metropolitan Archdiocese of Udine |
| 1818.05.01 | Italy | Diocese of Feltre lost territory to Diocese of Padova and Diocese of Trento |
| 1818.05.01 | Italy | Metropolitan Archdiocese of Ferrara exchanged territory with Diocese of Adria |
| 1818.05.01 | Italy | Diocese of Padova exchanged territory with Diocese of Vicenza |
| 1818.05.01 | Italy | Metropolitan Archdiocese of Ravenna lost territory to Diocese of Adria |
| 1818.05.01 | Italy | Diocese of Torcello was suppressed (to Patriarchal See of Venezia) |
| 1818.05.01 | Italy | Metropolitan Archdiocese of Udine was demoted as Diocese of Udine (gained territory from Patriarchal See of Venezia, lost territory to Diocese of Ceneda and Diocese of Treviso, exchanged territory with Diocese of Concordia and became suffragan of Venezia) |
| 1818.05.01 | Italy | Patriarchal See of Venezia lost territory to Diocese of Ceneda, Diocese of Gorizia e Gradisca and Diocese of Udine |
| 1818.05.01 | Italy | Diocese of Verona lost territory to Diocese of Padova |
| 1818.05.01 | Italy | Diocese of Vicenza exchanged territory with Diocese of Padova |
| 1818.05.02 | Germany | Diocese of Konstanz lost territory to Diocese of Brixen |
| 1818.05.02 |  Switzerland | Diocese of Chur lost territory to Diocese of Brixen |
| 1818.06.08 |  Mauritius,  British Indian Ocean Territory | Apostolic Vicariate of Cape of Good Hope and adjacent territories was established (from Diocese of Tomé and Territorial Prelature of Mozambique) |
| 1818.06.27 | Italy | Archpresbyterate nullius of Acquaviva delle Fonti was suppressed (to Metropolitan Archdiocese of Bari e Canosa) |
| 1818.06.27 | Italy | Diocese of Alessano was suppressed (to Diocese of Ugento) |
| 1818.06.27 | Italy | Diocese of Alife was suppressed (to Diocese of Telese) |
| 1818.06.27 | Italy | Metropolitan Archdiocese of Amalfi was demoted as Archdiocese of Amalfi (gained territory from the suppressed Diocese of Minori, Diocese of Ravello and Diocese of Scala and without ecclesiastical province) |
| 1818.06.27 | Italy | Diocese of Belcastro was suppressed (to Metropolitan Archdiocese of Santa Severina) |
| 1818.06.27 | Italy | Diocese of Bitetto was suppressed (to Metropolitan Archdiocese of Bari e Canosa) |
| 1818.06.27 | Italy | Diocese of Caiazzo was suppressed (to Diocese of Caserta) |
| 1818.06.27 | Italy | Diocese of Campli was suppressed (to Diocese of Teramo) |
| 1818.06.27 | Italy | Diocese of Canne was suppressed (to Metropolitan Archdiocese of Trani) |
| 1818.06.27 | Italy | Territorial Prelature of Canosa was suppressed (to Diocese of Andria) |
| 1818.06.27 | Italy | Diocese of Capri was suppressed (to Metropolitan Archdiocese of Sorrento) |
| 1818.06.27 | Italy | Diocese of Carinola was suppressed (to Diocese of Sessa Aurunca) |
| 1818.06.27 | Italy | Diocese of Castro was suppressed (to Metropolitan Archdiocese of Otranto) |
| 1818.06.27 | Italy | Diocese of Cerenza was suppressed (to Diocese of Cariati) |
| 1818.06.27 | Italy | Diocese of Città Ducale was suppressed (to Diocese of L’Aquila) |
| 1818.06.27 | Italy | Metropolitan Archdiocese of Cosenza was demoted as Archdiocese of Cosenza (without ecclesiastical province) |
| 1818.06.27 | Italy | Diocese of Fondi was suppressed (to Diocese of Gaeta) |
| 1818.06.27 | Italy | Diocese of Frigento was suppressed (to Diocese of Avellino) |
| 1818.06.27 | Italy | Diocese of Giovinazzo was suppressed (to Diocese of Molfetta) |
| 1818.06.27 | Italy | Diocese of Guardialfiera was suppressed (to Diocese of Termoli) |
| 1818.06.27 | Italy | Diocese of Isola was suppressed (to Diocese of Crotone) |
| 1818.06.27 | Italy | Diocese of Lavello was suppressed (to Diocese of Venosa) |
| 1818.06.27 | Italy | Diocese of Lettere–Gragnano was suppressed (to Diocese of Castellammare di Stabia) |
| 1818.06.27 | Italy | Diocese of Martirano was suppressed (to Diocese of Nicastro) |
| 1818.06.27 | Italy | Diocese of Massa Lubrense was suppressed (to Metropolitan Archdiocese of Sorrento) |
| 1818.06.27 | Italy | Archdiocese of Matera was suppressed (to Metropolitan Archdiocese of Acerenza) |
| 1818.06.27 | Italy | Diocese of Minervino was suppressed (to Diocese of Andria) |
| 1818.06.27 | Italy | Diocese of Minori was suppressed (to Archdiocese of Amalfi) |
| 1818.06.27 | Italy | Diocese of Montecorvino was suppressed (to Diocese of Lucera) |
| 1818.06.27 | Italy | Diocese of Montemarano was suppressed (to Diocese of Nusco) |
| 1818.06.27 | Italy | Diocese of Monteverde was suppressed (to Metropolitan Archdiocese of Conza and Diocese of Sant’Angelo dei Lombardi) |
| 1818.06.27 | Italy | Diocese of Mottola was suppressed (to Diocese of Castellaneta) |
| 1818.06.27 | Italy | Metropolitan Archdiocese of Nazareth in Barletta was suppressed (to Metropolitan Archdiocese of Trani) |
| 1818.06.27 | Italy | Diocese of Nocera de’ Pagani was suppressed (to Diocese of Cava) |
| 1818.06.27 | Italy | Diocese of Ortona was suppressed (to Metropolitan Archdiocese of Lanciano) |
| 1818.06.27 | Italy | Diocese of Ostuni was suppressed (to Metropolitan Archdiocese of Brindisi) |
| 1818.06.27 | Italy | Diocese of Ravello was suppressed (to Archdiocese of Amalfi) |
| 1818.06.27 | Italy | Diocese of Sarno was suppressed (to Diocese of Cava) |
| 1818.06.27 | Italy | Diocese of Satriano was suppressed (to Metropolitan Archdiocese of Conza and Diocese of Campagna) |
| 1818.06.27 | Italy | Diocese of Scala was suppressed (to Archdiocese of Amalfi) |
| 1818.06.27 | Italy | Diocese of Strongoli was suppressed (to Diocese of Cariati) |
| 1818.06.27 | Italy | Diocese of Terlizzi was suppressed (to Diocese of Molfetta) |
| 1818.06.27 | Italy | Diocese of Trevico was suppressed (to Diocese of Lacedonia) |
| 1818.06.27 | Italy | Diocese of Umbriatico was suppressed (to Diocese of Cariati) |
| 1818.06.27 | Italy | Diocese of Venafro was suppressed (to Diocese of Isernia) |
| 1818.06.27 | Italy | Diocese of Vulturara was suppressed (to Diocese of Lucera) |
| 1818.06.28 | Italy | Diocese of Vico Equense was suppressed (to Metropolitan Archdiocese of Sorrento) |
| 1818.06.30 |  Poland | Metropolitan Archdiocese of Gniezno lost territory to Metropolitan Archdiocese of Warszawa |
| 1818.06.30 | Poland | Diocese of Kielce was suppressed (to Diocese of Kraków and to establish Diocese of Sandomierz) |
| 1818.06.30 | Poland | Diocese of Kraków lost territory to Metropolitan Archdiocese of Lviv and Diocese of Wrocław |
| 1818.06.30 | Poland | Diocese of Kujawy–Pomorze was renamed as Diocese of Kujawy–Kaliska (lost territory to Metropolitan Archdiocese of Gniezno, Diocese of Płock, Diocese of Poznań and Diocese of Wrocław and became suffragan of Warszawa) |
| 1818.06.30 | Poland | Diocese of Podlachia was established (from Diocese of Kraków, Diocese of Lublin, Diocese of Lutsk and Zytomierz, Diocese of Płock and Diocese of Poznań and became suffragan of Warszawa) |
| 1818.06.30 | Poland | Diocese of Płock lost territory to Metropolitan Archdiocese of Warszawa |
| 1818.06.30 | Poland | Diocese of Sandomierz was established (from Diocese of Kraków, from the suppressed Diocese of Kielce and became suffragan of Warszawa) |
| 1818.06.30 | Poland | Diocese of Sejny was established (from Diocese of Płock and from the suppressed Diocese of Wigry) |
| 1818.06.30 | Poland | Diocese of Warszawa was promoted as Metropolitan Archdiocese of Warszawa (with suffragan sees Kraków, Kujawy–Kaliska, Lublin, Płock, Podlachia and Sandomierz and gained territory from Metropolitan Archdiocese of Gniezno and Diocese of Płock) |
| 1818.06.30 | Poland | Diocese of Wigry was suppressed (to establish Diocese of Sejny) |
| 1818.07.27 | Italy | Diocese of Polignano was suppressed (to Diocese of Monopoli) |
| 1818.09.12 | Italy | Abbacy nullius of Asola was suppressed (to Diocese of Brescia and Diocese of Mantova) |
| 1818.09.22 |  Slovakia | Diocese of Prešov (Slovak Rite) was established (from Diocese of Mukačeve) |
