Early Middle Japanese (中古日本語, Chūko-Nihongo) is a stage of the Japanese language between 794 and 1185, which is known as the Heian period (平安時代). The successor to Old Japanese (上代日本語), it is also known as Late Old Japanese. However, the term "Early Middle Japanese" is preferred, as it is closer to Late Middle Japanese (中世日本語, after 1185) than to Old Japanese (before 794).

Old Japanese had borrowed and adapted the Chinese script to write Japanese. In Early Middle Japanese, two new scripts emerged: the kana scripts hiragana and katakana. That development simplified writing and brought about a new age in literature, with many classics such as The Tale of Genji, The Tale of the Bamboo Cutter, and The Tales of Ise.

Early Middle Japanese was written in three different ways. It was first recorded in Man'yōgana (万葉仮名), literally "ten thousand leaves borrowed labels", in reference to the Man'yōshū poetry anthology and the "borrowing" of the kanji characters as "labels" for the sounds of Japanese. Certain Chinese characters were borrowed to phonetically spell out Japanese sounds. Cursive handwriting gradually gave rise to the hiragana (平仮名, "flat/simple borrowed labels") and Buddhist shorthand practices of using pieces of kanji to denote the sounds then developed into the katakana (片仮名, "partial/piece borrowed labels").

It is worth noting that the man'yōgana in each cell only indicates one of many contemporary options for spelling each Japanese mora – in the table above, each chosen character is the direct origin of the corresponding modern hiragana. See also Hentaigana for a fuller description of how multiple hiragana could be used to spell a single sound. Also note that hiragana forms were not standardized at that time.

Although man'yōgana specify different kanji to represent voiced phonemes versus unvoiced phonemes, it is not until the Meiji period that we see standardized usage of the dakuten diacritic ゛ to explicitly mark voicing for hiragana and katakana.

Japan officially adopted simplified shinjitai (新字体, "new character forms") in 1946 as part of a round of orthographic reforms intended to improve literacy rates. The so-called kyūjitai (旧字体, "old character forms") are equivalent to Traditional Chinese characters, and these forms were the ones used in historical man'yōgana. Modern transcriptions of classical texts are predominantly written in shinjitai. To avoid unnecessary ambiguity, quotes from classical texts would be written in kyūjitai.

Additionally, there are many spelling differences between Modern Japanese and Early Middle Japanese even for the same word. For example, 万葉集 is spelled in modern Japanese hiragana as まんようしゅう (man'yōshū), while in Early Middle Japanese, this would have been まんえふしふ (man'yefushifu). Details on these spelling rules are helpful for understanding historical kana usage.

Major phonological changes were characteristic of the period.