Switzerland has the only square national flag among United Nations member states. Its flag is a red square bearing a bold white Greek cross, making it immediately distinctive among national flags.
The square design developed from medieval military banners used by the Old Swiss Confederacy. The white cross became a recognizable Swiss symbol during the 14th and 15th centuries, while the modern red-and-white flag was standardized in the 19th century. Switzerland formally adopted the square federal flag in 1889.
The cross is not the same as the Red Cross emblem, although the humanitarian organization’s symbol was deliberately created by reversing the colors of the Swiss flag. Switzerland’s civil and merchant ensigns are exceptions to the square rule: they use a rectangular version of the design for ships and commercial purposes.
Vatican City also has a square flag, which causes a frequent objection to this question. However, Vatican City is a non-member observer state rather than a United Nations member state. Among full UN members, Switzerland is the sole country with a square national flag.