The Three Kingdoms of Korea, with Goguryeo and Buyeo in blue.
The Goguryeo language was a Koreanic language spoken in the ancient kingdom of Goguryeo (37 BCE – 668 CE), one of the Three Kingdoms of Korea. The language is also known as Old Koguryo, Koguryoic, and Koguryoan.
It is unknown except for a small number of words, which mostly suggest that it was similar but not identical to the language of Silla, and may have been influenced by the Tungusic languages. Striking similarities between later Baekje and Goguryeo can also be found, which is consistent with the legends that describe Baekje being founded by the sons of Goguryeo's founder. The Goguryeo names for government posts are mostly similar to those of Baekje and Silla.
Many experts today, including Ki-moon Lee, S. Robert Ramsey, Alexander Vovin, John Whitman and Marshall Unger, believe that Goguryeo language was a member of the Koreanic language family that either spread from southern Manchuria and the Korean peninsula at earlier times or expanded from Goguryeo during the Three Kingdoms of Korea period to Baekje and Silla by Goguryeo migrants. It is argued that the connections of Goguryeo toponyms to Japonic may be due to earlier languages of southern Korea prior to the expansion of Koreanic languages. Words of Goguryeo origin can be found in Middle Korean (early 10th to late 14th century).
Christopher I. Beckwith links Buyeo, Goguryeo, and Baekje with Japonic languages. This work was criticized for serious methodological flaws, such as rejecting mainstream reconstruction of Chinese and Japanese and using his own instead. Juha Janhunen argues that it is possible that a Tungusic-speaking elite ruled Goguryeo and Balhae, or Goguryeo language could have been an Amuric language related to today's Nivkh language isolate. 
- Balhae language
- List of Korea-related topics
- History of the Korean language
- Buyeo languages
- Old Korean
- Koreanic languages
- Korean influence on Japanese culture
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- Fan Ye, Book of the Later Han, volume 85; the Dongyi Liezhuan
- Wei Shou, Book of Wei, volume 100; the Liezhuan 88, the Wuji
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- Vovin, Alexander (2013). "From Koguryo to Tamna: Slowly riding to the South with speakers of Proto-Korean". Korean Linguistics. 15 (2): 222–240.
- Lee, Ki-Moon; Ramsey, S. Robert (2011). A History of the Korean language. Cambridge: Cambridge University Press. ISBN 978-0-521-66189-8.
- Whitman, John (2011). "Northeast Asian Linguistic Ecology and the Advent of Rice Agriculture in Korea and Japan". Rice. 4 (3-4): 149–158.
- Unger, J. Marshall (2009). The role of contact in the origins of the Japanese and Korean languages. Honolulu: University of Hawaii Press. ISBN 978-0-8248-3279-7.
- Toh Soo Hee, About Early Paekche Language Mistaken as Being Koguryo Language, Ch'ungnam University
- Beckwith, Christopher (2005). "The Ethnolinguistic History of the Early Korean Peninsula Region: Japanese-Koguryic and other Languages in the Koguryo, Paekche, and Silla kingdoms". Journal of Inner and East Asian Studies. 2–2: 33–64.
- Pellard, Thomas (2005). "Koguryo, the Language of Japan's Continental Relatives: An Introduction to the Historical-Comparative Study of the Japanese-Kgouryoic Languages with a Preliminary Description of Archaic Northeastern Middle Chinese (review)". Korean Studies. 29: 167–170.
- Pozzi & Janhunen & Weiers 2006, p. 109
- Janhunen, Juha (2005). "The Lost Languages of Koguryo". Journal of Inner and East Asian Studies. 2–2: 65–86.
- Pozzi, Alessandra; Janhunen, Juha Antero; Weiers, Michael, eds. (2006). Tumen Jalafun Jecen Aku: Manchu Studies in Honour of Giovanni Stary. Volume 20 of Tunguso Sibirica. Giovanni Stary. Otto Harrassowitz Verlag. ISBN 344705378X. Retrieved 1 April 2013.
- Beckwith, C. I. (2004). Koguryo, the language of Japan's continental relatives: an introduction to the historical-comparative study of the Japanese Koguryoic languages with a preliminary description of Archaic northeastern Middle Chinese. Brill's Japanese studies library, v. 21. Boston: Brill. ISBN 90-04-13949-4
- Beckwith (2006). "Methodological Observations on Some Recent Studies of the Early Ethnolinguistic History of Korea and Vicinity." Altai Hakpo 2006, 16: 199-234.
- Beckwith (2006). "The Ethnolinguistic History of the Early Korean Peninsula Region: Japanese-Koguryoic and Other Languages in the Koguryo, Paekche, and Silla Kingdoms." Journal of Inner and East Asian Studies, 2006, Vol. 2-2: 34-64.
- Beckwith (2007): Koguryo, the Language of Japan's Continental Relatives: An Introduction to the Historical-Comparative Study of the Japanese-Koguryoic Languages, with a Preliminary Description of Archaic Northeastern Middle Chinese. Brill Academic Publishers, 2004. ISBN 90-04-13949-4. Second edition, 2007. ISBN 90-04-16025-6
- Alexander Vovin (2005). "Koguryǒ and Paekche: Different Languages or Dialects of Old Korean?" Journal of Inner and East Asian Studies, 2005, Vol. 2-2: 108-140.