The small group of Hachijō or Hachijōjima dialects are the most divergent form of Japanese or form an independent fourth branch of Japonic.^[2] They are spoken on the southern Izu Islands south of Tokyo, Hachijō Island and the smaller Aogashima, as well as on the Daitō Islands of Okinawa Prefecture, which were settled from Hachijō in the Meiji period. Based on the criterion of mutual intelligibility, Hachijō may be considered a distinct Japonic language.

Hachijō dialects retain ancient Eastern Japanese features, as recorded in the 8th-century Man'yōshū. There are also lexical similarities with the dialects of Kyushu and even the Ryukyuan languages; it is not clear if these indicate the southern Izu islands were settled from that region, if they are loans brought by sailors traveling among the southern islands, or if they might be independent retentions of Old Japanese.^[3]

The dialect of Aogashima is quite distinct. There are also numerous dialects on Hachijō Island, with the speech of nearly every village distinct. There may be a few speakers left of the dialect of Little Hachijō Island, which was abandoned in 1969.

Hachijō uses the be-verb aru with all subjects, without the animate–inanimate (iru–aru) distinction made on the mainland. It also distinguishes between the attributive form (連体形 rentaikei) and terminal form (終止形 shūshikei) of verbs and adjectives, a distinction that existed in Early Middle Japanese but has all but vanished from the modern language.

Hachijō preserves a number of phrases that have been otherwise lost in the rest of Japan, such as まぐれる magureru for standard 気絶する kizetsu suru 'to faint, pass out'. There are also words which occur in standard Japanese, but with different meanings:^[4]