A torii is a traditional Japanese gate most commonly found at the entrance of or within a Shinto shrine, where it symbolically marks the transition from the mundane to the sacred.

The presence of a torii at the entrance is usually the simplest way to identify Shinto shrines, and a small torii icon represents them on Japanese road maps.

The first appearance of torii gates in Japan can be reliably pinpointed to at least the mid-Heian period; they are mentioned in a text written in 922. The oldest existing stone torii was built in the 12th century and belongs to a Hachiman Shrine in Yamagata prefecture. The oldest existing wooden torii is a ryōbu torii (see description below) at Kubō Hachiman Shrine in Yamanashi prefecture built in 1535.

Torii gates were traditionally made from wood or stone, but today they can be also made of reinforced concrete, copper, stainless steel or other materials. They are usually either unpainted or painted vermilion with a black upper lintel. Shrines of Inari, the kami of fertility and industry, typically have many torii because those who have been successful in business often donate torii in gratitude. Fushimi Inari-taisha in Kyoto has thousands of such torii, each bearing the donor's name.

The function of a torii is to mark the entrance to a sacred space. For this reason, the road leading to a Shinto shrine (sandō) is almost always straddled by one or more torii, which are therefore the easiest way to distinguish a shrine from a Buddhist temple. If the sandō passes under multiple torii, the outer of them is called ichi no torii (一の鳥居, first torii). The following ones, closer to the shrine, are usually called, in order, ni no torii (二の鳥居, second torii) and san no torii (三の鳥居, third torii). Other torii can be found farther into the shrine to represent increasing levels of holiness as one nears the inner sanctuary (honden), core of the shrine.[18] Also, because of the strong relationship between Shinto shrines and the Japanese Imperial family, a torii stands also in front of the tomb of each Emperor.

The origins of the torii are unknown and there are several different theories on the subject, none of which has gained universal acceptance. Because the use of symbolic gates is widespread in Asia—such structures can be found for example in India, China, Thailand, Korea, and within Nicobarese and Shompen villages—historians believe they may be an imported tradition. 

They may, for example, have originated in India from the torana gates in the monastery of Sanchi in central India. According to this theory, the torana was adopted by Shingon Buddhism founder Kūkai, who used it to demarcate the sacred space used for the homa ceremony. The hypothesis arose in the 19th and 20th centuries due to similarities in structure and name between the two gates. Linguistic and historical objections have now emerged, but no conclusion has yet been reached.