Date | Country | Change |
| 1866 |  Italy | Diocese of Reggio Emilia lost territory to Diocese of Guastalla |
| 1866 | Italy | Abbacy nullius of Santissimo Salvatore (Italo-Albanese Rite) was suppressed (to Metropolitan Archdiocese of Messina) |
| 1866 |  Switzerland | Diocese of Chur lost territory to Diocese of Sankt Gallen |
| 1866.03.20 |  Norway,  Canada,  Denmark,  Faroe Islands,  Greenland,  Iceland,  Scotland,  Sweden | Apostolic Prefecture of North Pole was renamed as Apostolic Prefecture of North Pole |
| 1866.07.25 |  Algeria | Diocese of Alger–Iulia Caesarea was promoted as Metropolitan Archdiocese of Alger–Iulia Caesarea (with suffragan sees Constantine and Oran and lost territory to establish Diocese of Constantine and Diocese of Oran) |
| 1866.07.25 | Algeria | Diocese of Constantine was restored (from Diocese of Alger–Iulia Caesarea, became suffragan of Alger–Iulia Caesarea and depended on Sacred Consistorial Congregation) |
| 1866.07.25 | Algeria | Diocese of Oran was established (from Diocese of Alger–Iulia Caesarea, became suffragan of Alger–Iulia Caesarea and depended on Sacred Consistorial Congregation) |
| 1866.11.01 |  Australia,  Norfolk Island | Metropolitan Archdiocese of Sydney lost territory to Diocese of Maitland |
| 1866.11.27 |  Myanmar | Apostolic Vicariate of Central Burma was established (from Apostolic Vicariate of Burma) |
| 1866.11.27 | Myanmar | Apostolic Vicariate of Eastern Burma was established (from Apostolic Vicariate of Burma) |
| 1866.11.27 | Myanmar | Apostolic Vicariate of Burma was renamed as Apostolic Vicariate of Southwestern Burma (lost territory to establish Apostolic Vicariate of Central Burma and Apostolic Vicariate of Eastern Burma) |
