The dialects (方言, hōgen) of the Japanese language fall into two primary clades, Eastern (including modern capital Tokyo) and Western (including old capital Kyoto), with the dialects of Kyushu and Hachijō Island often distinguished as additional branches, the latter perhaps the most divergent of all. The Ryukyuan languages of Okinawa Prefecture and the southern islands of Kagoshima Prefecture form a separate branch of the Japonic family, and are not Japanese dialects, although they are sometimes referred to as such.

Japan with its numerous islands and mountains has the ideal setting for developing many dialects.

Regional variants of Japanese have been confirmed since the Old Japanese era. The Man'yōshū, the oldest existing collection of Japanese poetry, includes poems written in dialects of the capital (Nara) and eastern Japan, but other dialects were not recorded. The compiler included azuma uta ("eastern songs") that show that eastern dialect traits were distinct from the western dialect of Nara. It is not clear if the capital of Nara entertained the idea of a standard dialect, however, they had an understanding which dialect should be regarded as the standard one, the dialect of the capital.

The recorded features of eastern dialects were rarely inherited by modern dialects, except for a few language islands such as Hachijo Island. In the Early Middle Japanese era, there were only vague records such as "rural dialects are crude". However, since the Late Middle Japanese era, features of regional dialects had been recorded in some books, for example Arte da Lingoa de Iapam, and the recorded features were fairly similar to modern dialects. In these works, recorded by the Christian missionaries in Japan, they regard the true colloquial Japanese as the one used by the court nobles in Kyōto. Other indications for the Kyōto dialect to be considered the standard dialect at that time are glossaries of local dialects that list the Kyōto equivalent for local expressions.

The variety of Japanese dialects developed markedly during the Early Modern Japanese era (Edo period) because many feudal lords restricted the movement of people to and from other fiefs. Some isoglosses agree with old borders of han, especially in Tohoku and Kyushu. Nevertheless, even with the political capital being moved to Edo (i.e. Tōkyō) the status of the Kyōto dialect was not threatened immediately as it was still the cultural and economic center that dominated Japan. This dominance waned as Edo began to assert more political and economic force and made investments in its cultural development. At the end of the eighteenth century the Japanese that was spoken in Edo was regarded as standard as all glossaries from this period use the Edo dialect for local expressions.

In the Meiji period the Tōkyō dialect was assuming the role of a standard dialect that was used between different regions to communicate with each other. The Meiji government set policies in place to spread the concept of 標準語 (hyōjun-go; "standard language"). One of the main goals was to be an equal to the western world and the unification of the language was a part to achieve this. For the hyōjun-go the speech of the Tōkyō middle class served as a model. The Ministry of Education at this time made text books in the new standard language and fostered an inferiority complex in the minds of those who spoke in dialects besides the Tōkyō dialect. One example is a student who was forced to wear a "dialect tag" around the neck. From the 1940s to the 1960s, the period of Shōwa nationalism and the post-war economic miracle, the push for the replacement of regional varieties with Standard Japanese reached its peak.

After World War II, the concept of 共通語 (Kyōtsū-go; "common language") was introduced, which differed from the concept of the standard language insofar that it is heavily influenced by the standard language but it retains dialectal traits. Across Japan, the 'common language' productively used in everyday speech can differ from region to region but it is still mutually intelligible.

Now Standard Japanese has spread throughout the nation, and traditional regional varieties are declining because of education, television, expansion of traffic, urban concentration, etc, in a process known as dialect levelling. However, regional varieties have not been completely replaced with Standard Japanese. The spread of Standard Japanese means the regional varieties are now valued as "nostalgic", "heart-warming" and markers of "precious local identity", and many speakers of regional dialects have gradually overcome their sense of inferiority regarding their natural way of speaking. The contact between regional varieties and Standard Japanese creates new regional speech forms among young people, such as Okinawan Japanese.