The Northern Thai people, also known as Tai Yuan (Thai: ไทยวน, [tʰaj˧ juan˧]) or Khon Mueang (Northern Thai: ᨤᩫ᩠ᨶᨾᩮᩬᩥᨦ, คนเมือง pronounced [kʰon˧ mɯaŋ˧]), are a Tai ethnic group, native to nine provinces in Northern Thailand, principally in the area of the former kingdom of Lan Na. As a Tai group, they are closely related to Tai Lü and Tai Khün with regards to common culture, language and history in contrast to Thailand's dominant Thai ethnic group (referred to as Siamese or Central Thai). There are approximately 6 million Tai Yuan. Most of them live in Northern Thailand, with a small minority 29,442 (2005 census) living across the border in Bokeo Province of Laos. Their language is called Northern Thai, Lanna or Kham Mueang.

The Northern Thai people refer to themselves as khon muang, meaning "people of the (cultivated) land," "people of our community" or "society" (mueang is a central term in Tai languages that has a broad meaning and is essential to the social structure of Tai peoples). With that name, they historically identified themselves as the inhabitants of the alluvial plains, river valleys and plateaus of their native area, where they lived in local communities, called muang, and cultivated rice on paddy fields. That distinguished them from the indigenous peoples of the area ("hill tribes"), like the Lua', who lived in the wooded mountains practicing slash-and-burn agriculture. Membership of the Northern Thai was therefore defined by lifestyle, rather than genetics. At the same time, it was a term of dissociation from the Burmese and Siamese, who held suzerainty over the Lanna Kingdom for centuries and who were not "people of our muang".

For the same reasons, the khon muang call themselves kammuang or kham muang in which kam means language or word, and muang means town; hence, the meaning "town language", contrasts those of the many hill tribes in the surrounding mountainous areas.

The exonym Tai Yuan is likely derived from Sanskrit yavana meaning "stranger," which itself comes from the name of the Greek tribe of the Ionians, or from Pali yonaka. In everyday speech, "Tai" prefixed to some location is understood as meaning "Tai person" of that place. Predecessors to the term "Yuan" were used by the Chams, Vietnamese, and Khmer as exonyms for other ethnic groups in the region. The Khmer form is still used today, as a pejorative exonym for the Vietnamese. The Burmese historically referred to the Northern Thai people as the Yun (ယွန်း), which in turn is now the Burmese word for "lacquerware."

The British colonial rulers in neighbouring Burma (now Myanmar) referred to them as Siamese Shan to distinguish them from the Shan proper, whom they called Burmese Shan.

Until the 20th century, the Siamese considered Northern Thais to be "Lao", due to linguistic and cultural differences, or more specifically as Lao phung dam, or black-bellied Lao because of their tradition of tattooing their abdomens (phung), which contrasted with the Lao to their east, who did not have that custom. According to Jit Bhumisak, a prominent Thai historian, Northern Thais consider themselves Tai-Thai and do not refer to themselves as Lao. That is reflected in various inscriptions in which the term "Thai-Tai" is used to refer to themselves. The term "Lao" is seen as an insult by Northern Thais, as it is associated with a savage and uncivilized culture. Therefore, the use of the term Khon Muang is a way for Northern Thais to assert their distinct identity and cultural heritage and to distance themselves from the negative connotations of the term "Lao".

The Northern Thais also call Central Thais "Thai" and add the word "South" to refer to Southern Thais or "Southerners." However, they do not use the term Tai/Thai to refer to other ethnicities that interact more closely with La Nna society, such as Tai Yai, Tai Khoen, Tai Lue people, which reflects the fact that they see themselves and those ethnic groups as distinct entities.

According to a shared legend amongst various Tai peoples, a possibly-mythical king, Khun Borom Rachathiriat of Mueang Then begot several sons that settled and ruled other mueang, or city-states, across Southeast Asia and southern China. Descended from ancient peoples known to the Chinese as the Yue and the Ai Lao, the Tai tribes began migrating into South-East Asia by the beginning of the 1st millennium, but large-scale migrations took place between the 7th and the 13th centuries AD, especially from what is now Sipsongbanna, Yunnan Province and Guangxi. The possible reasons for Tai migration include pressures from Han Chinese expansion, Mongol invasions, finding suitable land for wet rice cultivation and the fall of the states in which the Tais inhabited. < According to linguistic and other historical evidence, Tai-speaking tribes migrated southwestward to the modern territories of Laos and Thailand from Guangxi sometime between the 8th and the 10th centuries. The Tai assimilated or pushed out indigenous Austroasiatic Mon–Khmer peoples, and settled on the fringes of the Indianized kingdoms of the Mon and Khmer Empire. The blending of peoples and the influx of Indian philosophy, religion, language, culture and customs via and alongside some Austroasiatic element enriched the culture of the Tai peoples, but the Tais remained in contact with the other Tai mueang.