| Conventional Long Name: | Principality of Ratzeburg |
| Common Name: | Ratzeburg |
| Today: | Germany |
| Empire: | Holy Roman Empire |
| Year Start: | 1648 |
| Year End: | 1815 |
| Event Start: | Peace of Westphalia – secularised to Mecklenburg-Güstrow |
| Date Start: | 24 October |
| Event End: | Subsumed by Grand Duchy of Mecklenburg-Strelitz |
| Event1: | Treaty of Hamburg – to Mecklenburg-Strelitz |
| Date Event1: | 8 March 1701 |
| P1: | Prince-Bishopric of Ratzeburg |
| Image Map Caption: | The Principality of Ratzeburg (left-hand yellow territory) within the Duchy of Mecklenburg |
The Principality of Ratzeburg (German: Fürstentum Ratzeburg) is a former state, existing from 1648 to 1918. It belonged to the imperially immediate territory of the Duchy of Mecklenburg within the Holy Roman Empire. It was formed from the territory of the former Prince-Bishopric of Ratzeburg, which was secularised to Mecklenburg-Güstrow in the 1648 Peace of Westphalia. Following the death of the last duke in 1695, the territories of Mecklenburg-Güstrow were split up in the 1701 Treaty of Hamburg (the third partition of Mecklenburg), which created the semi-ducal states of Mecklenburg-Schwerin and Mecklenburg-Strelitz, with the latter made up of the Principality of Ratzeburg and the Lordship of Stargard. Its territories remained exclaves of Mecklenburg-Strelitz through its later iterations – the Grand Duchy from 1815 and Free State from 1918 – before the bulk of it was finally incorporated into the unified state of Mecklenburg in 1934 as part of German: {{ill|Landkreis Schönberg|de; its small exclaves were incorporated into German: [[Kreis Herzogtum Lauenburg]] of Schleswig-Holstein. Most of the Principality is now within the state of Mecklenburg-Vorpommern.
Geschichte des Bisthums Ratzeburg. F. Aschenfeldt, Lübeck 1835 (Volltext).