For the Masamune as a video game weapon, see Masamune (video game weapon)

Masamune, also known as Goro Nyudo, is widely recognized as Japan's greatest swordsmith. No exact dates are known for Masamune's life, but it is generally agreed that he made most of his swords in the early-to-mid 1300's. He created swords, known as katana in Japanese, in the Soshu tradition, with only the cutting edge of the blade being tempered. Masamune's swords have a reputation for superior hardness and quality, remarkable in a period where the steel necessary for swords was often impure.

Swords created by Masamune often are referred to with the smith's name (much the same way that other pieces of artwork are), often with a name for the individual sword as well. The Honjo Masamune , a symbol of the Tokugawa shogunate and passed down from one shogun to another, is perhaps the best known Masamune sword.

The swords of Masamune are often contrasted with those of Muramasa, another Japanese swordsmith. Muramasa has alternatively been described as a full contemporary of Masamune, Masamune's student, and as having lived too late to have met Masamune. Often, Muramasa's blades are described as bloodthirsty or evil while Masamune's are considered the mark of an internally peaceful and calm warrior. The most famous legend of the difference between the weapons of the two swordmakers is based on what happens when one of each sword is placed in a slow stream. Muramasa's sword was able to cut a leaf cleanly in half, while another leaf abruptly changed course and slid by Masamune's sword without touching it.