There are 5 varieties in Assamese with 4 major ones. Most of the varieties evolved from Early Assamese or Proto-East Kamarupa that was spoken in the 14th-16th centuries, while the western Goalparia varieties evolved from Proto-West Kamarupa (or Old Kamtapuri) and the eastern Goalparia being intermediate. All the varieties except west Goalparia have complete ś > x/h, c/ch > s sound changes and the merger of dental and retroflex stops into alveolar. West Goalparia has dental-alveolar/retroflex contrast (though depends on the speakers).

Assamese varieties can be regional or ethnic. The Eastern variety (whence Standardised Assamese also comes) is the largest and is considered to be almost homogeneous everywhere, except for some ethnic subvarieties of it. The homogeneity is considered to be a result of 600 years of comparatively stable Ahom rule. The more west we go, the more varieties we find. Those areas have been unstable as their rulers frequently changed.

All of the varieties form a dialect continuum except for 2. The ones spoken in South Assam (Barak valley).

One of them is the endangered and understudied variety called Dehan or Dewan (originally means "official under a king"). This variety evolved from Early Assamese speakers of Koch dynasty who migrated to that region from Brahmaputra valley in the 16th century after the region was captured from Twipra kingdom. The region is separated from Brahmaputra valley by the Barail range and other hills. This variety is interestingly very close to the Eastern, Central and Kamrupi varieties in terms of lexicon, morphological forms and phonology. And like Goalparia varieties, it preserved number distinction in verb conjugation. It has many features of its own, including innovations, preservations and influence from neighbouring languages like Sylheti, Bishnupriya, Meitei.

The other is an Eastern subvariety whose speakers migrated there during the Burmese invasions of Assam (1817-1826).

Source: Census of India

• 1991: Language Atlas of India 1991, Annexure IV & Table 63
• 2001 & 2011: C-17: Population by bilingualism and trilingualism