A tachi (太刀) is a type of sabre-like traditionally made Japanese sword (nihonto) worn by the samurai class of feudal Japan. Tachi and uchigatana ("katana") generally differ in length, degree of curvature, and how they were worn when sheathed, the latter depending on the location of the mei (銘), or signature, on the tang. The tachi style of swords preceded the development of the katana, which was not mentioned by name until near the end of the twelfth century. Tachi were the mainstream Japanese swords of the Kotō period between 900 and 1596. Even after the Muromachi period (1336–1573), when katana became the mainstream, tachi were often worn by high-ranking samurai.

The production of swords in Japan is divided into specific time periods:

The predecessor of the Japanese sword has been called warabitetō  [ja]. In the middle of the Heian period (794–1185), samurai improved on the warabitetō to develop kenukigata-tachi  [ja] (early Japanese sword). To be more precise, it is thought that the Emishi improved the warabitetō and developed Kenukigata-warabitetō  [ja] with a hole in the hilt and kenukigatatō  [ja] without decorations on the tip of the hilt, and the samurai developed kenukigata-tachi based on these swords. Kenukigata-tachi, which was developed in the first half of the 10th century, has a three-dimensional cross-sectional shape of an elongated pentagonal or hexagonal blade called shinogi-zukuri and a gently curved single-edged blade, typical features of Japanese swords. There is no wooden hilt attached to kenukigata-tachi, and the tang (nakago), integrated with the blade, is directly gripped and used. The term kenukigata is derived from the fact the central part of tang is hollowed in the shape of ancient Japanese tweezers (kenuki).

In the tachi developed after kenukigata-tachi, a structure in which the hilt is fixed to the tang (nakago) with a pin called mekugi was adopted. As a result, a sword with three basic external elements of Japanese swords, the cross-sectional shape of shinogi-zukuri, a gently curved single-edged blade, and the structure of nakago, was completed. Its shape may reflect the changing form of warfare in Japan. Cavalry were now the dominant fighting unit, and the older straight chokutō were unsuitable for fighting from horseback. The curved sword is a far more efficient weapon wielded by a warrior on horseback; the curve of the blade adds considerably to the downward force of a cutting action. According to historian Karl Friday, before the 13th century, there are no written references or drawings showing swords of any kind were used from horseback.^{[full citation needed]} However, According to Yoshikazu Kondo, bow and arrows were certainly the main weapons used in cavalry battles, but from around the Genpei War in the 12th century, the use of tachi on horseback increased. Early models had uneven curves with the deepest part of the curve at the hilt. As eras changed, the center of the curve tended to move up the blade.

By the 11th century during the Heian period, tachi were exported to neighboring countries in Asia. For example, in the poem "The Song of Japanese Swords", Ouyang Xiu, a statesman of the Song dynasty in China, described Japanese swords as follows: "It is a treasured sword with a scabbard made of fragrant wood covered with fish skin, decorated with brass and copper, and capable of exorcising evil spirits. It is imported at a great cost."

From the Heian period (794–1185), ordinary samurai wore swords of the style called kurourusi tachi (kokushitsu no tachi, 黒漆太刀), which meant black lacquer tachi. The hilt of a tachi is wrapped in leather or ray skin, and it is wrapped with black thread or leather cord, and the scabbard is coated with black lacquer. On the other hand, court nobles wore tachi decorated with precisely carved metal and jewels for ceremonial purposes. High-ranking court nobles wore swords of the style called kazari tachi or kaza tachi (飾太刀, 飾剣), which meant decorative tachi, and lower-ranking court nobles wore simplified kazatachi swords of the style called hosodachi (細太刀), which meant thin tachi. The kazatachi and hosodachi worn by nobles were initially straight like a chokutō, but since the Kamakura period they have had a gentle curve under the influence of tachi. Since tachi worn by court nobles were for ceremonial use, they generally had an iron plate instead of a blade.

In the Kamakura period (1185–1333), high-ranking samurai wore hyogo gusari tachi (hyogo kusari no tachi, 兵庫鎖太刀), which meant a sword with chains in the arsenal. The scabbard of the tachi was covered with a gilt copper plate and hung by chains at the waist. At the end of the Kamakura period, simplified hyogo gusari tachi came to be made as an offering to the kami of Shinto shrines and fell out of use as weapons. On the other hand, in the Kamakura period, there was a type of tachi called hirumaki tachi (蛭巻太刀) with a scabbard covered with metal, which was used as a weapon until the Muromachi period. The meaning was a sword wrapped around a leech, and its feature was that a thin metal plate was spirally wrapped around the scabbard, so it was both sturdy and decorative, and chains were not used to hang the scabbard around the waist.

The Mongol invasions of Japan in the 13th century during the Kamakura period facilitated a change in the designs of Japanese swords. The swordsmiths of the Sōshū school represented by Masamune studied ruined tachi – broken or bent in battle – to develop new production methods, and create innovative swords. They forged the blade using a combination of soft and hard steel to optimize the temperature and timing of the heating and cooling of the blade, resulting in a lighter and very robust blade. They also made the curve of the blade gentle, lengthened the tip linearly, widened the width from the cutting edge to the opposite side of the blade, and thinned the cross section to improve the penetration and cutting ability of the blade.