Zagajnik was formerly part of the German Empire. In the German Empire, the place was called Zagajnik.
The place is now called Zagajnik and belongs to Poland.
| Historical place name | Country | Administration | Time |
|---|---|---|---|
| Zagajnik | Russian Empire | Mlava | 1914 |
| Zagajnik | Poland | Mława | 1919 |
| Zagajnik | German Empire | Mielau | 1939 |
| Zagajnik | Poland | Żuromin | 1945 |
| Zagajnik | Poland | Ciechanów | 1992 |