The largest of the ethnic groups in Cambodia are the Khmer, who comprise 95.8% of the total population and primarily inhabit the lowland Mekong subregion and the central plains. The Khmer historically have lived near the lower Mekong River in a contiguous arc that runs from the southern Khorat Plateau where modern-day Thailand, Laos and Cambodia meet in the northeast, stretching southwest through the lands surrounding Tonle Sap lake to the Cardamom Mountains, then continues back southeast to the mouth of the Mekong River in southeastern Vietnam.
Ethnic groups in Cambodia other than the politically and socially dominant Khmer are classified as either "indigenous ethnic minorities" or "non-indigenous ethnic minorities". The indigenous ethnic minorities, more commonly collectively referred to as the Khmer Loeu ("upland Khmer"), constitute the majority in the remote mountainous provinces of Ratanakiri, Mondulkiri and Stung Treng and are present in substantial numbers in Kratie Province.
Approximately 17-21 separate ethnic groups, most of whom speak Austroasiatic languages related to Khmer, are included in the Khmer Loeu designation, including the Kuy and Tampuan people. These peoples are considered to be the aboriginal inhabitants of the land by the Cambodian authorities. Two of these highland groups, the Rade and the Jarai, are Chamic peoples who speak Austronesian languages descended from ancient Cham. These indigenous ethnic minorities haven't integrated into Khmer culture and follow their traditional animist beliefs.
The non-indigenous ethnic minorities include immigrants and their descendants who live among the Khmer and have adopted, at least nominally, Khmer culture and language. The three groups most often included are the Chinese Cambodians, Vietnamese and Cham peoples. The Chinese have immigrated to Cambodia from different regions of China throughout Cambodia's history, integrating into Cambodian society and today Chinese Cambodians or Cambodians of mixed Sino-Khmer ancestry dominate the business community, politics and the media. The Cham are descendants of refugees from the various wars of the historical kingdom of Champa. The Cham live amongst the Khmer in the central plains but in contrast to the Khmer who are Theravada Buddhists, the vast majority of Cham follow Islam.
There are also small numbers of other minority groups. Tai peoples in Cambodia include the Lao along the Mekong at the northeast border, Thai (urban and rural), and the culturally Burmese Kola, who have visibly influenced the culture of Pailin Province. Even smaller numbers of recent Hmong immigrants reside along the Lao border and various Burmese peoples have immigrated to the capital, Phnom Penh.
Ethnic Khmer
The Khmers are one of the oldest ethnic groups in the area, having filtered into Southeast Asia around the same time as the Mon. Most archaeologists and linguists, and other specialists like Sinologists and crop experts, believe they arrived no later than 2000 BCE (over four thousand years ago) bringing with them the practice of agriculture and in particular the cultivation of rice. They were the builders of the later Khmer Empire which dominated Southeast Asia for six centuries beginning in 802 CE, and now form the mainstream of political, cultural, and economic Cambodia.
The Khmers developed the first alphabet still in use in Southeast Asia which in turn gave birth to the later Thai and Lao scripts. The Khmers are considered by most archaeologists and ethnologists to be indigenous to the contiguous regions of Isan, southernmost Laos, Cambodia and Southern Vietnam. That is to say the Khmer have historically been a lowland people who lived close to one of the tributaries of the Mekong.
The Khmers see themselves as being one ethnicity connected through language, history and culture, but divided into three main subgroups based on national origin. The Khmer of Cambodia speak a dialect of the Khmer language. The Northern Khmer (Khmer Surin) are ethnic indigenous Khmers whose lands once belonged to the Khmer Empire but have since become part of Thailand. The Northern Khmer also speak the Isan language fluently.
Maintaining close relations with the Khmer of Cambodia, some now reside in Cambodia as a result of marriage. Similarly, the Khmer Krom are indigenous Khmers living in the regions of the former Khmer Empire that are now part of Vietnam. Fluent in both their particular dialect of Khmer and in Vietnamese, many have fled to Cambodia as a result of persecution and forced assimilation by Vietnam.
All three varieties of Khmer are mutually intelligible. While the Khmer language of Cambodia proper is non-tonal, surrounding languages such as Thai, Vietnamese and Lao are all highly tonal and have thus affected the dialects of Northern Khmer and Khmer Krom.
Vietnamese Cambodians
Vietnamese-Cambodians also called The "Khmer Youn People", They are mostly located in Southern-Eastern Cambodia, bordering Vietnam, and accounted for 6.25% of the Cambodian total population.
Prior to the Cambodian Civil War, the Vietnamese were the most populous ethnic minority in Cambodia, with an estimated 450,000 living in provinces concentrated in the southeast of the country adjacent to the Mekong Delta. Vietnamese Cambodians also lived further upstream along the shores of the Tonlé Sap. During the war, the Vietnamese community in Cambodia was "entirely eradicated". As of the 2019 census, speakers of Vietnamese accounted for 0.5% Cambodia's 15.6 million people.
Most of these came to the country as a result of the post-civil war Vietnamese invasion and occupation of Cambodia, during which time the Vietnamese-installed government of Cambodia (the People's Republic of Kampuchea) relied heavily on Vietnam for the rebuilding of its economy. Following the 1993 withdrawal of Vietnamese troops, the government of modern Cambodia maintained close ties with Vietnam and Vietnamese-backed ventures came to the country looking to capitalize on the new market. In addition to these mostly urban immigrants, some villagers cross the border illegally, fleeing impoverished rural conditions in Vietnam's socialist one-party state hoping for better opportunities in Cambodia.
Although the Vietic languages are also within the Austroasiatic language family like Khmer, there are very few cultural connections between the Vietnamese peoples because the early Khmers were part of Greater India while the Vietnamese are part of the East Asian cultural sphere and adopted Chinese literary culture.
Ethnic tensions between the two can be traced to the Post-Angkor Period (from the 16th to 19th centuries), during which time a nascent Vietnam and Thailand each attempted to vassalize a weakened post-Angkor Cambodia, and effectively dominate all of Indochina. Control over Cambodia during this, its weakest point, fluctuated between Thailand and Vietnam. Vietnam unlike Thailand, wanted Cambodia to adopt Vietnamese governmental practices, dress, and language. The Khmers resented and resisted until they were incorporated into the colonial French Indochina.
During the colonial period, the French brought over Vietnamese middlemen to administer the local Cambodian government, causing further resentment and anti-Vietnamese sentiment that endures to the present.
Due to the long history of the two countries, there is a significant amount of Cambodians of mixed Vietnamese and Khmer ancestry. Most of these Vietnamese-Cambodians no longer speak Vietnamese and have assimilated into Khmer society and identify as Khmer. They have engaged primarily in aquaculture in the Mekong Delta of the southeast.
Chinese
Chinese Cambodians are approximately 0.6% of the population. Jayavarman II unified the Chenla states and established the Khmer Empire (or Kambuja) which declared himself Chakravartin (Universal Monarch) at Phnom Kulenin the early 9th century, with Chinese influence playing a role in trade and documentation rather than mass settlement during this specific timeframe. Because the Khmer society was strictly agrarian and hierarchical, the "merchant" class was often filled by the Chinese traditionally and matrilineal (tracing descent through the mother), these "Sino-Khmers" were quickly absorbed into the Khmer social fabric, often becoming part of the local administrative elite, they did not leave their own monuments due to their knowledge comes almost exclusively from Chinese Imperial Annals like the Book of Tang. Most Chinese are descended from 19th–20th century settlers who came in search of trade and commerce opportunities during the time of the French protectorate. Waves of Chinese migration have been recorded as early as the twelfth century during the time of the Khmer Empire. Most are urban dwellers, engaged primarily in commerce.
The Chinese in Cambodia belong to five major linguistic groups, the largest of which is the Teochew accounting for about 60%, followed by the Cantonese (20%), the Hokkien (7%), and the Hakka and the Hainanese (4% each).
Intermarriage between the Chinese and Khmers has been common, in which case they would often assimilate into mainstream Khmer society, retaining few Chinese customs. Much of the Chinese population dwindled under Pol Pot during the Cambodian Civil War. The Chinese were not specifically targets for extermination, but suffered the same brutal treatment faced by the ethnic Khmers during the period.
Tai
Tai peoples present in Cambodia include the Thai, Lao, Tai Phuan, Nyaw, Shan, and the Kula (, , also known by the Thai designation, "Kula", and, historically, by the Burmese name, "Tongsoo"). Thai speakers in Cambodia amount to less than .01% of the population.
In modern times, Thai people are mainly to be found in the capital, Phnom Penh, primarily as families of either the diplomatic mission or representatives of Thai companies doing business in Cambodia. The northwestern provinces were administratively a part of Thailand for most of the period from the 1431 fall of Angkor until the 20th century French Protectorate. Descendants of the Thais and many people of Khmer-Thai ancestry reside in these provinces, but have mostly assimilated to Khmer culture and language and are indistinguishable from their fellow Khmer villagers.
Siamese Cambodians
Siamese Cambodians are Cambodian citizens with Thai ethnicity. They are mostly located in Koh Kong Province, in western Cambodia, bordering Thailand, and accounted for 25% of the province's total population in 2008..The ethnic Thai population numbered in the tens of thousands before the Cambodian Civil War but in 1975 over five thousand fled across the border into Thailand while another 35 thousand were systematically evacuated from Koh Kong Province and many were killed as spies.
Lao
Lao people reside in the far northeast of the country, inhabiting villages scattered among the hill tribes and along the Mekong and its tributaries in the mountainous regions near the Lao border. Historically part of Funan and later the heartland of the pre-Angkorian Khmer Chenla Kingdom, the region now encompassed by Stung Treng, Ratanakiri and parts of Preah Vihear, Kratie and Mondulkiri Provinces were all but abandoned by the Khmer during the Middle Period as the Khmer Empire waned and the population moved south to more strategic and defensible positions.
The area fell under the rule of the Lao kingdom of Lan Xang in the 14th century and remained part of successive Lao kingdoms until, in 1904 during the French Indochina period, the region was returned to Cambodian administration. Consequently, notwithstanding the relatively recent immigration of Khmers back to the area, as of 2010, ethnic Lao constituted more than half the population of Stung Treng, a substantial number (up to 10%) in Ratanakiri and smaller communities in Preah Vihear and Mondulkiri.