Dozens of dialects are spoken in Japan. The profusion is due to the mountainous island terrain and Japan's long history of both external and internal isolation. Dialects typically differ in terms of pitch accent, inflectional morphology, vocabulary, particle usage, and pronunciation. Some even differ in vowel and consonant inventories, although this is uncommon.

Dialects from less central regions, such as the Tōhoku or Tsushima dialect may be unintelligible to speakers from other parts of the country. The dialect used in Kagoshima in southern Kyūshū is famous for being unintelligible not only to speakers of standard Japanese but to speakers of nearby dialects elsewhere in Kyūshū as well. Kagoshima dialect is 84% cognate with standard Tokyo dialect. Kansai-ben, a group of dialects from west-central Japan, is spoken by many Japanese; the Osaka dialect in particular is associated with comedy, and many entertainers use Osaka dialect phrases solely for humor value.

The Ryukyuan languages are spoken in the Ryukyu Islands. Not only is each language unintelligible to Japanese speakers, but most are unintelligible to those who speak other Ryukyuan languages. Due to the close relationship of Ryukyuan and Japanese, they are still sometimes said to be only dialects of one language, but modern scholars consider them to be separate languages.

Recently, Standard Japanese has become prevalent nationwide, due not only to television and radio, but also to increased mobility within Japan due to its system of roads, railways, and airports. Young people usually speak their local dialect and the standard language, though in most cases, the local dialect is influenced by the standard, and regional versions of "standard" Japanese have local-dialect influence.