Onigiri (お握り or 御握り), also known as omusubi (お結び) or nigirimeshi (握り飯), is a Japanese rice ball made from white rice. It is usually formed into triangular or cylindrical shapes, and wrapped in nori (seaweed). Onigiri traditionally have sour or salty fillings such as umeboshi (pickled Chinese plum), salted salmon, katsuobushi (smoked and fermented bonito), kombu, tarako or mentaiko (pollock roe), or takanazuke (pickled Japanese giant red mustard greens). Because it is easily portable and eaten by hand, onigiri has been used as portable food or bento from ancient times to the present day. Originally, it was used as a way to use and store left-over rice, but it later became a regular meal. Many Japanese convenience stores and supermarkets stock onigiri with various fillings and flavors. It has become so mainstream that it is even served in izakayas and sit-down restaurants. There are even specialized shops which only sell onigiri to take out. Due to the popularity of this trend in Japan, onigiri has become a popular staple in Japanese restaurants worldwide.

Onigiri is not a form of sushi and should not be confused with the type of sushi called nigirizushi or simply nigiri. Onigiri is made with plain rice (sometimes lightly salted), while sushi is made of rice with vinegar, sugar, and salt. Onigiri makes rice portable and easy to eat as well as preserving it, while sushi originated as a way of preserving fish.

On November 12, 1987, lumps of carbonized grains of rice, thought to be riceballs, were excavated from a building belonging to the Yayoi period (2000 years ago) in the Sugitani Chanobatake Ruins in Ishikawa Prefecture. The carbonized rice had traces which revealed that it was formed by human hands, thus it was initially documented as "the oldest onigiri." In subsequent research, it was thought to be steamed and grilled, rather than boiled like today's rice, similar to another dish called chimaki. Since then, it has been academically called the "chimaki-shaped carbonized rice lumps (チマキ状炭化米塊)".

In Nakanoto, there is a replica of the relic on display at the roadside station Orihime-no-sato Nakanoto.

Before the use of chopsticks became widespread, in the Nara period, rice was often rolled into a small ball so that it could be easily picked up. In the Heian period, rice was made into small rectangular shapes known as tonjiki so that they could be piled onto a plate and easily eaten. At that time, onigiri were called tonjiki and often consumed at outdoor picnic lunches.

The first incarnation of the word onigiri is attested in the Hitachi no Kuni Fudoki (Hitachi Province Gazette), dated to 721 AD. In it, the word used is nigiri-ihi (握飯) or "crumpled rice":

In Murasaki Shikibu's 11th-century diary Murasaki Shikibu Nikki, she writes of people eating tonjiki rice balls. Other writings, dating back as far as the seventeenth century, state that many samurai stored rice balls wrapped in bamboo sheath as a quick lunchtime meal during war.

From the Kamakura period to the early Edo period, onigiri was used as a quick meal. This made sense as cooks simply had to think about making enough onigiri and did not have to concern themselves with serving. These onigiri were simply balls of rice flavored with salt. Nori did not become widely available until the Genroku era (1688–1704) of the mid-Edo period, when the farming of nori and fashioning it into sheets became widespread.^{[citation needed]}