The longest samurai sword is the Ōdachi, also known as the nodachi. The Ōdachi is a type of Japanese sword characterized by its exceptionally long blade. While there is no fixed length for an Ōdachi, it typically refers to a sword with a blade length of over 90 centimeters (around 35 inches) and sometimes exceeding 100 centimeters (39 inches).

The Ōdachi was primarily used by samurai warriors during the feudal period of Japan. Due to its size, the Ōdachi was wielded with both hands and was considered a ceremonial or battlefield weapon rather than a practical sword for everyday combat. The long blade allowed for wide and sweeping strikes, capable of inflicting significant damage on multiple opponents.

It's worth noting that the Ōdachi fell out of favor as the Japanese battlefield tactics shifted towards more compact and versatile weapons. Surviving examples of the Ōdachi can be found in museums or private collections, and they are appreciated for their historical and artistic value.