After World War I, upon the Greater Poland Uprising of 1918/19 and according to the Treaty of Versailles, the town was part of the small, mostwestern part of Greater Poland which remained part of Weimar Germany.
From 1922 these lands, located close to the border with the Second Polish Republic, were administrated as the Prussian Province of Posen-West Prussia.
Meseritz became the seat of the Landeshauptmann governor, until in 1938 the province was dissolved and Meseritz was incorporated into the Province of Brandenburg.
From 1934 to 1938, the Nazi administration had the Festungsfront Oder-Warthe-Bogen fortifications erected in the south of the town.
During World War II, in Obrawalde (present-day Obrzyce within the city limits), from 1942 to 1945, Germans murdered over 10,000 people, mostly mentally ill and disabled, as part of the Aktion T4, but also prisoners of war and political opponents.

In the late days of World War II, Meseritz was occupied by Red Army forces in the course of the Vistula–Oder Offensive on 31 January 1945.
Left to the Republic of Poland, it was incorporated into Poznań Voivodeship on July 7.
The town's re-incorporation to Poland was confirmed with the Potsdam Agreement and the implementation of the Oder–Neisse line.
While the Polish autochthonous minority was allowed to remain in Międzyrzecz, the German speaking majority was expelled.
Poles of whom a part had been expelled or left from former Kresy territories, annexed by the Soviet Union, re-settled the town.
After World War I, upon the Greater Poland Uprising of 1918/19 and according to the Treaty of Versailles, the town was part of the small, mostwestern part of Greater Poland which remained part of Weimar Germany. From 1922 these lands, located close to the border with the Second Polish Republic, were administrated as the Prussian Province of Posen-West Prussia. Meseritz became the seat of the Landeshauptmann governor, until in 1938 the province was dissolved and Meseritz was incorporated into the Province of Brandenburg. From 1934 to 1938, the Nazi administration had the Festungsfront Oder-Warthe-Bogen fortifications erected in the south of the town. During World War II, in Obrawalde (present-day Obrzyce within the city limits), from 1942 to 1945, Germans murdered over 10,000 people, mostly mentally ill and disabled, as part of the Aktion T4, but also prisoners of war and political opponents.
In the late days of World War II, Meseritz was occupied by Red Army forces in the course of the Vistula–Oder Offensive on 31 January 1945. Left to the Republic of Poland, it was incorporated into Poznań Voivodeship on July 7. The town's re-incorporation to Poland was confirmed with the Potsdam Agreement and the implementation of the Oder–Neisse line. While the Polish autochthonous minority was allowed to remain in Międzyrzecz, the German speaking majority was expelled. Poles of whom a part had been expelled or left from former Kresy territories, annexed by the Soviet Union, re-settled the town.
Międzyrzecz