An ōdachi (大太刀, large/great sword) or nodachi (野太刀, field sword) is a type of traditionally made Japanese sword (日本刀, nihontō) used by the samurai class of feudal Japan. The Chinese equivalent of this type of sword in terms of weight and length is the miaodao or the earlier zhanmadao, and the Western battlefield equivalent (though less similar) is the Zweihänder.

To qualify as an ōdachi, the sword in question would have a blade length of around 3 shaku (90.9 cm (35.8 in)). However, as with most terms in Japanese sword arts, there is no exact definition of the size of an ōdachi.

The character for ō (大) means "big" or "great"; no (野) means "field". The dachi here (太刀) is simply the voiced compounding version of the term tachi (太刀, great sword), the older style of sword that predates the katana. The second character in tachi, 刀, is the Chinese character for "blade" (see also dāo), and is also the same character used to spell katana (刀) and the tō in nihontō (日本刀 "Japanese sword").

The word tachi itself is derived as the stem or noun form of verb tatsu (断つ; "to cut off"). The kanji spelling is an example of jukujikun, applying a semantically based kanji spelling without regard to the usual phonetic values of the characters.

Ōdachi became popular in Kamakura period (1185－1333). Until the middle of the Kamakura period, high-ranking samurai mainly fought on horseback with yumi (bows), but as group battles by foot soldiers increased from the late Kamakura period, the importance of weapons possessed by those who did not have horses and did not have sufficient training in bows increased. Until then, they mainly used naginata with a long handle, but they also started to use ōdachi. The Kamakura period was the first time that samurai ruled Japan, and powerful men were valued, and those who wanted to show off the honor of being a warrior preferred to use ōdachi.

In the Nanboku-chō period in the 14th century, huge Japanese swords such as ōdachi were at their peak. The reason for this is thought to be that the conditions for making a practical large-sized sword were established due to the nationwide spread of strong and sharp swords of the Sōshū school. In the case of ōdachi whose blade was 150 cm (59 in) long, it was impossible to draw a sword from the scabbard on the waist, so people carried it on their back or had their servants carry it. Large naginata and kanabō were also popular in this period. However, infantry units gradually came to be equipped with yari (spears) in addition to naginata, and because ōdachi was disadvantageous for mountain battles and surprise attacks, and only a few powerful men could use it effectively, this trend ended for a short time. Furthermore, from the Sengoku period in the latter part of the Muromachi period to the Azuchi-Momoyama period, as tactics shifted to fighting with yari and tanegashima (guns) by a large group of infantry, ōdachi became even more obsolete. As ōdachi became useless, it was often replaced with a tachi and katana.

Even so, sengoku-daimyo in the Sengoku period dared to equip their own troops with ōdachi in order to show off their strength and bravery. Uesugi Kenshin had men more than six shaku (approx. 182 cm (72 in)) tall equipped with an ōdachi guard around his horse. The Asakura clan made a troop called Rikishizei (力士勢) equip with an ōdachi with a blade length of 5 shaku (approx. 152 cm (60 in)), and fought well against the troop of Oda Nobunaga in the Battle of Anegawa.

The ōdachi was used as a weapon, but because of its magnificent appearance, it was often used as an offering to kami, a Shinto shrine. For example, Ōyamazumi Shrine, which is said to be a treasure house of Japanese swords and armor, is dedicated to the national treasure Ōdachi, which was dedicated by Emperor Go-Murakami, and ōdachi, which was dedicated by Ōmori Naoharu and killed Kusunoki Masashige.