A shinai (竹刀) is a Japanese sword typically made of bamboo used for practice and competition in kendō. Shinai are also used in other martial arts, but may be styled differently from kendō shinai, and represented with different characters. The light, soft wood used in a shinai distinguishes it from other wooden swords such as a bokuto (木刀), usually called a bokken (木剣) outside Japan, which is generally made of heavier, sturdier wood.

The earliest use of a bamboo weapon to train with instead of a sword is credited to Kamiizumi Nobutsuna (1508–1572?) of the Shinkage-ryū. The modern shinai, with four slats of bamboo, is generally credited to Nakanishi Chuzo Tsugutate (died 1801) of Nakanishi-ha Ittō-ryū. The shinai was developed in an effort to reduce the number of practitioners being seriously injured during practice, making a practice weapon that was less dangerous than bokutō (木刀), the hard wooden swords they were previously using. This is also the motivation behind the development of bōgu (防具), the armour that protects the kendoka.

Due to its lighter weight compared to a bokken or a metal katana, a shinai can be wielded in a fashion that allows quicker strikes than would be practicable with a heavier sword. This also makes shinai popular in producing movies and television shows; a prop shinai with a thin metal covering can look similar to a metal sword, while allowing cinematic, showy strikes that are nevertheless fairly safe to perform.

The word "shinai" is derived from the verb shinau (撓う), meaning "to bend, to flex", and was originally short for shinai-take (flexible bamboo). Shinai is written with the kanji 竹刀, meaning "bamboo sword", and is an irregular kanji reading.

In kendo, it is most common to use a single shinai, sometimes called ittō (一刀) style. Some kendōka choose to use two shinai. This kendō style is usually called ni-tō (二刀), a style that has its roots in the two-sword schools of swordsmanship such as Hyōhō Niten Ichi-ryū. A ni-tō combatant uses a long shinai called the daitō (大刀), which is usually held in the right hand, and a shorter shinai, called the shōtō (小刀), which is usually held in the left hand. The holding position can be switched, however, with the daitō in the left hand and the shōtō in the right. The daitō is shorter and lighter than a shinai used in the ittō style of kendō. Specifications for shinai used in kendō competitions that follow the International Kendo Federation (FIK) rules are below.

Sizes and style of shinai vary. For example, an adult may be able to use a shinai that is too heavy for a younger person, so shinai with different sizes and characteristics are made. Shinai are available in many styles and balances. A shinai should not be confused with a bokutō, which has a much more similar shape and length to a Japanese sword and is made from a single piece of wood. However, both shinai and bokken are used in kendo.

The slats of a shinai are usually made from dried bamboo. Some may also be treated by smoking them, or soaking them in resin. Shinai slats are also made of carbon fibre, reinforced resin, or other approved alternative materials.

The shinai comprises four slats known as take (竹), which are held together by three leather fittings: a hilt (tsuka), or handle wrapping (tsuka-gawa (柄皮)); a fitting at the tip (saki-gawa (先皮)) and a leather strip (naka-yui (中結)) that binds the four slats. The parts are all secured with a string (tsuru (弦)).