Middle Korean is the period in the history of the Korean language succeeding Old Korean and yielding to Early Modern Korean in the late 16th century. The boundary between the Old and Middle periods is traditionally identified with the establishment of Goryeo in 918 and the associated transition of the prestige dialect from the Southeast to the center of the peninsula, but some scholars have argued for the Mongol invasions of Korea in the mid-13th century. Middle Korean is divided into Early and Late periods corresponding to Goryeo (until 1392) and Joseon respectively.

It is difficult to extract linguistic information from texts of the Early period, which are written with Chinese characters (called Hanja in Korean). The situation was transformed in 1446 by the introduction of the Hangul alphabet, so that Late Middle Korean provides the pivotal data for the history of Korean.

Middle Korean is traditionally taken to span the period from the establishment of Goryeo in 918, when the political centre moved from Gyeongju in the southeast to Kaesong in the central west, up to the start of the Imjin War in 1592. The boundary between early and late periods is variously taken as the beginning of the Joseon period in 1392 or the promulgation of Hangul in 1446. The Joseon capital was only a short distance away in Hanyang (modern Seoul), so any language change would have been minimal, but the introduction of Hangul dramatically changed the documentation of the language.

Until the late 19th century, most formal writing in Korea, including government documents, scholarship and much literature, was written in Classical Chinese. Before the 15th century, the little writing in Korean was done using cumbersome systems using Chinese characters, such as idu and hyangchal. Thus Early Middle Korean, like Old Korean before it, is sparsely documented.

Before the 1970s, the key sources for EMK were a few wordlists.

In 1973, close examination of a Buddhist sutra from the Goryeo period revealed faint interlinear annotations with simplified Chinese characters indicating how the Chinese text could be read as Korean. More examples of kugyŏl ('oral embellishment') were discovered, particularly in the 1990s. Many of the kugyŏl characters were abbreviated, and some of them are identical in form and value to symbols in the Japanese katakana syllabary, though the historical relationship between the two is not yet clear. An even more subtle method known as kakp'il (각필; 角筆; lit. 'stylus') annotations was discovered in 2000. This consists of dots and lines made with a stylus at various positions around a character, with their interpretation depending on the position at which they were placed. Both forms of annotation contain little phonological information, but are valuable sources on grammatical markers.

The kugyŏl used after the Mongol invasions of Korea in the mid-13th century differs from that used before in style and grammar. Nam Pung-hyun suggests that the language changed due to the disruption of the invasions and occupation, and the period before should be considered Late Old Korean rather than part of Middle Korean.

The introduction of the Hangul alphabet in 1446 revolutionized the description of the language. The Hunminjeongeum ('The Correct/Proper Sounds for the Instruction of the People') and later texts describe the phonology and morphology of the language with great detail and precision. Earlier forms of the language must be reconstructed by comparing fragmentary evidence with LMK descriptions. These works are not as informative regarding Korean syntax, as they tend to use a stilted style influenced by Classical Chinese. The best examples of colloquial Korean are the translations in foreign-language textbooks produced by the Joseon Bureau of Interpreters.