The Mon–Khmer languages are a language family of Southeast Asia. Together with the Munda languages of India, they are one of the two traditional primary branches of the Austro-Asiatic family. However, several recent classifications have abandoned this dichotomy, either reducing the scope of Mon–Khmer (Diffloth 2005) or breaking it up entirely, effectively reclassifying Munda as a branch of Mon–Khmer (Sidwell 2009). (See Austro-Asiatic languages for details.)

The reconstructed ancestor of the Mon–Khmer languages is Proto-Mon–Khmer.

The classic classification of Mon–Khmer is that of Diffloth (1974), now abandoned by Diffloth himself. (See Austroasiatic languages for details.) The only proper classification in recent years is that of Diffloth (2005), though the evidence has not been published and so cannot be evaluated by other researchers. Diffloth (2005) breaks up his 1974 conception of Mon–Khmer, resulting in Khasi–Khmuic (see) and Nuclear Mon–Khmer:

Sidwell (2005) casts doubt on Diffloth's Vieto-Katuic hypothesis, saying that the evidence is ambiguous, and that it is not clear where Katuic belongs in the family. Using lexicostatistics, Sidwell (2009) sees no strong evidence for grouping the branches of Mon–Khmer, and abandons the distinction between Mon–Khmer and Austroasiatic, resulting in a family with potentially thirteen primary branches.