Date | Country | Change |
| 1821 |  Italy | Diocese of Reggio Emilia lost territory to Diocese of Carpi |
| 1821.01.30 |  Switzerland | Diocese of Lausanne was renamed as Diocese of Lausanne et Genève |
| 1821.05.14 | Italy | Diocese of Ostuni was restored (from Metropolitan Archdiocese of Brindisi and became suffragan of Brindisi) |
| 1821.06.19 |  USA | Diocese of Cincinnati was established (from Diocese of Bardstown and became suffragan of Baltimore) |
| 1821.07.16 |  Belgium | Diocese of Liège exchanged territory with Metropolitan Archdiocese of Köln |
| 1821.07.16 |  Germany | Diocese of Aachen was suppressed (to Metropolitan Archdiocese of Köln, Diocese of Münster and Diocese of Trier) |
| 1821.07.16 | Germany | Diocese of Corvey was suppressed (to Diocese of Paderborn) |
| 1821.07.16 | Germany | Metropolitan Archdiocese of Köln lost territory to Diocese of Münster, Diocese of Paderborn and Diocese of Trier |
| 1821.07.16 | Germany | Diocese of Mainz lost territory to Diocese of Paderborn |
| 1821.07.16 | Germany | Diocese of Münster exchanged territory with Diocese of Osnabrück |
| 1821.07.16 | Germany | Apostolic Vicariate of Nordic Missions lost territory to Diocese of Münster, Diocese of Osnabrück, Diocese of Paderborn and Diocese of Wrocław |
| 1821.07.16 | Germany | Diocese of Osnabrück exchanged territory with Diocese of Münster |
| 1821.07.16 | Germany | Diocese of Speyer exchanged territory with Diocese of Mainz |
| 1821.07.16 |  Poland | Diocese of Chełmno lost territory to Diocese of Warmia |
| 1821.07.16 | Poland | Metropolitan Archdiocese of Gniezno lost territory to Diocese of Chełmno |
| 1821.07.16 | Poland | Diocese of Poznań was promoted as Metropolitan Archdiocese of Poznań (gained territory from Diocese of Wrocław) |
| 1821.07.16 | Poland | Diocese of Wrocław lost territory to Metropolitan Archdiocese of Poznań |
| 1821.07.16 |  Russia | Diocese of Sambia was suppressed (to Diocese of Warmia) |
| 1821.07.20 | Poland | Diocese of Sejny was renamed as Diocese of Augustów |
| 1821.08.16 | Germany | Metropolitan Archdiocese of Freiburg im Breisgau was established (from Diocese of Mainz, Diocese of Speyer, Diocese of Strasbourg, former Diocese of Worms and Diocese of Würzburg, from the suppressed Diocese of Konstanz and with suffragan sees Fulda, Limburg, Mainz and Rottenburg) |
| 1821.08.16 | Germany | Diocese of Konstanz was suppressed (to establish Metropolitan Archdiocese of Freiburg im Breisgau and Diocese of Rottenburg) |
| 1821.08.16 | Germany | Diocese of Limburg was established (from Metropolitan Archdiocese of Köln, Diocese of Mainz and Diocese of Trier and became suffragan of Freiburg im Breisgau) |
| 1821.08.16 | Germany | Diocese of Mainz lost territory to Diocese of Fulda |
| 1821.08.16 | Germany | Diocese of Rottenburg was established (from the suppressed Diocese of Konstanz and became suffragan of Freiburg im Breisgau) |
| 1821.08.16 | Germany | Diocese of Trier exchanged territory with Diocese of Mainz |
| 1821.09.15 | Italy | Diocese of Norcia was restored (from Diocese of Spoleto and without ecclesiastical province) |
| 1821.09.15 | Italy | Diocese of Spoleto was promoted as Archdiocese of Spoleto (lost territory to establish Diocese of Norcia) |
| 1821.09.20 | Poland | Diocese of Tyniec was restored (from Diocese of Kraków and became suffragan of Lviv) |
