Denmark has the world's oldest continuously used national flag, according to Guinness World Records.
Denmark's flag is called the Dannebrog. It is a red field bearing a white Scandinavian cross that extends to the edges, with the vertical bar shifted toward the hoist. The design belongs to the Nordic cross tradition and influenced the flags of several other Nordic countries.
A medieval legend says the flag fell from the sky during the Battle of Valdemar in Estonia in 1219. Historians treat that story as a later tradition rather than securely documented evidence for the flag's origin. The flag's use is firmly documented from later centuries, and Denmark's government formally regulated its appearance in modern times.
“Oldest” can mean different things in flag discussions: oldest design, oldest surviving flag, or longest continuous national use. Denmark's record refers specifically to continuous use as a national flag, not to proof that every detail of the medieval legend is historical.