Muramasa (村正; born before 1501), commonly known as Sengo Muramasa (千子村正), was a famous swordsmith who founded the Muramasa school and lived during the Muromachi period (14th to 16th centuries) in Kuwana, Ise Province, Japan (current Kuwana, Mie).

In spite of their original reputation as fine blades favored by the shōgun Tokugawa Ieyasu and his vassals, the katana swords made by Muramasa gradually became a symbol of the anti-Tokugawa movement. Furthermore, in lore and popular culture from the 18th century, the swords have been regarded as yōtō (妖刀; "wicked katana").

Much like his unique reputation, Muramasa is known for some fairly unusual features in his work. These attributes are often called by terms prefixed with "Muramasa".

Although the school of Muramasa is extremely famous in popular culture, none of their swords are designated as a National Treasure or an Important Cultural Property.

Myōhō Muramasa (妙法村正; "Muramasa of the Sublime Dharma") is the only sword officially designated as an Important Artwork [ja]. Katana, length 66.4 cm, curvature 1.5 cm, bottom width 2.8 cm, shinogi-zukuri, iori-mune, and chū-kissaki nobi (see also Glossary of Japanese swords). The front side contains a sign of Muramasa and a mantra sign myōhō renge kyō (妙法蓮華経) (a mantra from Namu Myōhō Renge Kyō or the Lotus Sutra of Nichiren Buddhism). The back side contains a year sign 永正十年葵酉十月十三日 (13th day of the 10th month of Eishō 10, that is, 10 November 1513). It is highly probable that the date was chosen because the high priest Nichiren died on the 13th day of the 10th month of Kōan 5 (1282). Both sides contain beautiful engravings of Kurikara (Fudō Myō-ō's mythological sword empowered by a burning dragon). The style of the engravings is similar to those of the swordsmith Heianjō Nagayoshi, so some scholars suggest Muramasa studied under Nagayoshi. It is also silver-damascened with characters Nabeshin (鍋信), which suggests that the sword was once in possession of Nabeshima Katsushige (1580–1657), the first daimyō lord of Saga Domain. Later, this sword was given to Katsushige's son Nabeshima Motoshige, the first lord of Ogi Domain, and has been inherited by his successors.

Muramasa's students made excellent weapons too. Fujiwara Masazane, a disciple of Muramasa, forged Tonbokiri, one of the Three Great Spears of Japan. Masazane also forged a sword called Inoshishi-giri (猪切; "boar-slayer") whose name came from a legend that Sakai Tadatsugu killed a wild boar with this sword when accompanying Ieyasu in hunting.

The exact origin of the Muramasa school is unknown. The oldest extant sword equipped with both a name sign Muramasa and a date sign shows the year Bunki 1 (1501). Scholars, however, assert several swords signed with Muramasa (but without year signs) are slightly older than 1501 in light of their styles. It is generally thought that the school of Muramasa spanned at least three generations. It is hardly clear when the school disappeared, but some Muramasa swords contain the year sign Kanbun (1661–1673).

Lores in the late Muromachi period (early 16th century–1573) stated that Muramasa was a student of Masamune (c. 1300), the greatest swordsmith in Japan's history, and the Hon'ami family (family dynasty of swordpolishers and sword connoisseurs) commented that his floruit was the Jōji era (1362–1368). Scholars from the Azuchi–Momoyama period (1573–1600) to modern days, however, have dismissed the relationship of Masamune and Muramasa as fantasy because all of extant Muramasa swords are too new to support this theory. Another theory states that Muramasa was a student of Heianjō Nagayoshi, a prominent Kyoto swordsmith known for spears and engravings. The school of Masashige (正重), a notable branch of the Muramasa school, records Masashige died in 1456, so Muramasa was active before 1456 if we believe the record.