Eastern Old Japanese (abbreviated as EOJ; ) is a group of heterogenous varieties of Old Japanese, historically spoken in the east of Japan, in the area traditionally called Togoku or Azuma.
Eastern Old Japanese constitutes a branch of the Japanese subgroup of the Japonic languages (Insular Japonic), with the other varieties of Old Japanese, which all descend from proto-Japanese (separate from Proto-Ryukyuan, following the classification used by Kupchik (2011).
Eastern Old Japanese is mainly attested through poems collected in several anthologies written during the 8th century:
All this would give a total of 242 short poems and one long poem according to Alexander Vovin (2014).
This variety is geographically opposed to Western Old Japanese and Kyà «shà « Old Japanese. It was spoken to the east of Nara, the capital during the eponymous Nara period, approximately in the current Kantà  region, Chà «bu region and Tà Âhoku region, which was collectively referred to as Azuma.
Eastern Old Japanese was not a unified variety but rather a collection of dialects. Their demarcation differs depending on the author.
For example, Bjarke Frellesvig (2010) distinguishes three dialect areas:
He states that these dialects form a continuum with the varieties of Nara Old Japanese, with North Eastern Old Japanese constituting the most divergent variety. However, the majority of songs and poems do not have information on their provenance.
John Kupchik (2023) refers to all of these varieties as Azuma Old Japanese, consisting of two dialects: Töpo-Suruga Old Japanese in the three provinces of Frellesvig's southern area, and Eastern Old Japanese in the rest. The former dialect lacks attested loanwords in Ainu languages. He remarks on the differences in the spelling of the two varieties. In earlier work, he had separated the dialects of Shinano province as Central Old Japanese due to the absence of innovations shared with his Töpo-Suruga and Eastern Old Japanese groups.
Like the other Japonic languages, Eastern Old Japanese has a subjectâÂÂobjectâÂÂverb word order with a structure that includes a modifier at the beginning of the sentence, although there are exceptions. There are many suffixes, but unlike most SOV languages, there are also prefixes. Morphologically, it is principally an agglutinative language, but portmanteaus also exist.
The phonotactic structure of Eastern Old Japanese is strictly (C)V, without consonant gemination nor long vowels. Typically, vowel sequences contract rather than merge. The accent system is unknown.
There exists a correspondence between the Western Old Japanese *i and *u and the Eastern Old Japanese *(j)e and *o respectively, which is confirmed by the comparison of the three Japanese dialects, as well as the Ryukyuan languages. Thus, the Eastern Old Japanese vowel system would have been closer to that of Proto-Japonic than that of Western Old Japanese.
The Eastern Old Japanese lexicon is mainly inherited from Japonic languages. However, it is also contains Koreanic and Ainu loanwords, and only a few of Sinitic origin.
Other words are close to Japonic forms that appeared in later periods:
The dialects of Eastern Old Japanese were replaced by the Kyoto dialect of Early Middle Japanese, the descendant of Western Old Japanese during the Heian period (between the 8th and the 12th centuries). However, there are still modern traces of this variety:
According to Maner Lawton Thorpe (1983), the phonological correspondences of Eastern Old Japanese shared with the Ryukyuan languages could be explained by descent from a common language. Thus, he proposes the following phylogenetic tree:
Following his model, Western Old Japanese would have separated first, during the 4th and 5th centuries, then the Kyà «shà « branch would have separated three or four centuries later. Subsequently, Kantà  would have been populated by Japonic speakers directly from Kyà «shà «, without passing through central Japan.
However, Alexander Koji Makiyama (2015) finds the results of diachronic changes in Eastern Old Japanese such as in denasalization, fortition and vowel raising unconvincing in comparison with the Ryukyuan languages. In fact, he finds:
The hypothesis of linguistic contact or resemblance is therefore, at the current state of knowledge, only speculative. Thomas Pellard (2015) also considers this hypothesis as unproven.