Most scholars believe that Japonic was brought to the Japanese archipelago from the Korean peninsula with the Yayoi culture during the 1st millennium BC.

[3] Most scholars believe that Japonic was brought to northern Kyushu from the Korean peninsula around 700 to 300 BC by wet-rice farmers of the Yayoi culture and spread throughout the Japanese archipelago, replacing indigenous languages.

[4] There is fragmentary placename evidence that now-extinct Japonic languages were still spoken in central and southern parts of the Korean peninsula several centuries later.

[19] They also affected the sound system of the language by adding compound vowels, syllable-final nasals, and geminate consonants, which became separate morae.

[20] Most of the changes in morphology and syntax reflected in the modern language took place during the Late Middle Japanese period (13th to 16th centuries).

[21] Modern mainland Japanese dialects, spoken on Honshu, Kyushu, Shikoku, and Hokkaido, are generally grouped as follows:[22] The early capitals of Nara and Kyoto lay within the western area, and their Kansai dialect retained its prestige and influence long after the capital was moved to Edo (modern Tokyo) in 1603.

[28] The move from Kyushu to the Ryukyus may have occurred later and possibly coincided with the rapid expansion of the agricultural Gusuku culture in the 10th and 11th centuries.

[30] After the migration to the Ryukyus, there was limited influence from mainland Japan until the conquest of the Ryukyu Kingdom by the Satsuma Domain in 1609.

One proposal, adopted by the UNESCO Atlas of the World's Languages in Danger, has three subgroups, with the central "Kunigami" branch comprising varieties from Southern Amami to Northern Okinawan, based on similar vowel systems and patterns of lenition of stops.

[40] The internal classification by Elisabeth de Boer includes Ryukyuan as a deep subbranch of a Kyūshū–Ryūkyū branch:[41] She also proposes a branch consisting of the Izumo dialect (spoken on the northern coast of western Honshu) and the Tōhoku dialects (northern Honshu), which show similar developments in the pitch accent that she attributes to sea-borne contacts.

[42] There is fragmentary evidence suggesting that now-extinct Japonic languages were spoken in the central and southern parts of the Korean peninsula.

[5] The most-cited evidence comes from chapter 37 of the Samguk sagi (compiled in 1145), which contains a list of pronunciations and meanings of placenames in the former kingdom of Goguryeo.

As the pronunciations are given using Chinese characters, they are difficult to interpret, but several of those from central Korea, in the area south of the Han River captured from Baekje in the 5th century, seem to correspond to Japonic words.

[50] Samuel Elmo Martin, John Whitman, and others have proposed hundreds of possible cognates, with sound correspondences.

[66] Japonic languages, again like Ainu and Korean, are left-branching (or head-final), with a basic subject–object–verb word order, modifiers before nouns, and postpositions.

[70] Most Ryukyuan languages mark a clusivity distinction in plural (or dual) first-person pronouns, but no Mainland varieties do so.