Gorō Nyūdō Masamune (五郎入道正宗, Priest Gorō Masamune, c.1264–1343), was a medieval Japanese blacksmith widely acclaimed as Japan's greatest swordsmith. He created swords and daggers, known in Japanese as tachi and tantō, in the Sōshū school. However, many of his forged tachi were made into katana by cutting the tang (nakago) in later times. For this reason, his only existing works are katana and tantō. No exact dates are known for Masamune's life. It is generally agreed that he made most of his swords between 1288 and 1328. Some stories list his family name as Okazaki, but some experts believe this is a fabrication to enhance the standing of the Tokugawa family.
Masamune is believed to have worked in Sagami Province during the last part of the Kamakura period (1288–1328), and it is thought that he was trained by swordsmiths from Bizen and Yamashiro provinces, such as Saburo Kunimune, Awataguchi Kunitsuna and Shintōgo Kunimitsu. He was the father of Hikoshiro Sadamune, also a famous Sōshū master.
An award for swordsmiths called the Masamune Prize is awarded at the Japanese Sword Making Competition. Although not awarded every year, it is presented to a swordsmith who has created an exceptional work.