Which country has the oldest continuously used national flag?

Denmark

Answer: The Flag of Denmark holds the Guinness World Record for the oldest continuously used national flag. The current design of a white Scandinavian cross on a red back ground was adopted in 1625 and its square shape in 1748. In Denmark it is known as the ‘Dannebrog’ or ‘Danish cloth’.

Although Denmark was never part of the Roman Empire, similar designs were used by the Empire to represent provinces, as the white cross is symbolic of Christianity. The cross design was later adopted by other Nordic countries such as Sweden, Norway, Finland, and Iceland.