The Kra–Dai languages (/ˈkrɑː.daɪ/ KRAH-dy, also known as Tai–Kadai /ˈtaɪ.kəˌdaɪ/ TIE-kə-DYE and Daic /ˈdaɪ.ɪk/ DYE-ik), are a language family in mainland Southeast Asia, southern China, and northeastern India. All languages in the family are tonal, including Thai and Lao, the national languages of Thailand and Laos, respectively. Around 93 million people speak Kra–Dai languages; 60% of those speak Thai. Ethnologue lists 95 languages in the family, with 62 of these being in the Tai branch.

The name "Kra–Dai" was proposed by Weera Ostapirat (2000), as Kra and Dai are the reconstructed autonyms of the Kra and Tai branches, respectively. "Kra–Dai" has since been used by the majority of specialists working on Southeast Asian linguistics, including Norquest (2007), Pittayaporn (2009), Baxter & Sagart (2014), and Enfield & Comrie (2015).

The name "Tai–Kadai" is used in many references, as well as Ethnologue and Glottolog, but Ostapirat (2000) and others suggest that it is problematic and confusing, preferring the name "Kra–Dai" instead. "Tai–Kadai" comes from an obsolete bifurcation of the family into two branches, Tai and Kadai, which had first been proposed by Paul K. Benedict (1942). In 1942, Benedict placed three Kra languages (Gelao, Laqua (Qabiao), and Lachi) together with Hlai in a group that he called "Kadai", from ka, meaning "person" in Gelao and Laqua and dai, a form of a Hlai autonym. Benedict's (1942) "Kadai" group was based on his observation that Kra and Hlai languages have Austronesian-like numerals. However, this classification is now universally rejected as obsolete after Ostapirat (2000) demonstrated the coherence of the Kra branch, which does not subgroup with the Hlai branch as Benedict (1942) had proposed. "Kadai" is sometimes used to refer to the entire Kra–Dai family, including by Solnit (1988). Adding to the confusion, some other references^[which?] restrict the usage of "Kadai" to only the Kra branch of the family.

The name "Daic" is used by Roger Blench (2008).

James R. Chamberlain (2016) proposes that the Tai–Kadai (Kra–Dai) language family was formed as early as the 12th century BCE in the middle of the Yangtze basin, coinciding roughly with the establishment of the Chu fiefdom and the beginning of the Zhou dynasty. The high diversity of Kra–Dai languages in southern China, especially in Guizhou and Hainan, points to that being an origin of the Kra–Dai language family, founding the nations that later became Thailand and Laos in what had been Austroasiatic territory. Genetic and linguistic analyses show great homogeneity among Kra–Dai-speaking people in Thailand.

Although the position of Kra–Dai in relation to Austronesian is still contested, some propose that Kra–Dai and Austronesian are genetically connected. Weera Ostapirat (2005) sets out a series of regular sound correspondences between them, assuming a model of a primary split between the two; they would then be co-ordinate branches. Ostapirat (2013) continues to maintain that Kra–Dai and Austronesian are sister language families, based on certain phonological correspondences. On the other hand, Laurent Sagart (2008) proposes that Kra–Dai is a later form of what he calls "FATK" (Formosan Ancestor of Tai–Kadai) a branch of Austronesian belonging to the subgroup "Puluqic", developed in Taiwan, whose speakers migrated back to the mainland, to Guangdong, Hainan, and north Vietnam, around the second half of the 3rd millennium BCE. Upon their arrival in this region, they underwent linguistic contact with an unknown population, resulting in a partial relexification of FATK vocabulary. Erica Brindley (2015) supports Sagart's hypothesis, arguing that the radically different Kra-Dai history of migration to the mainland (as opposed to the Philippines for Proto-Austronesian) and extended contact with Austro-Asiatic and Sinitic speakers would make the relationship appear more distant. She also suggests that the presence of only the most basic Austronesian vocabulary in Kra-Dai makes this scenario of relexification more plausible.

Besides various concrete pieces of evidence for a Kra–Dai existence in present-day Guangdong, remnants of Kra–Dai languages spoken further north can be found in unearthed inscriptional materials and non-Han substrata in Min and Wu Chinese.^{[citation needed]}

Wolfgang Behr (2002, 2006, 2009, 2017) points out that most non-Sinitic words found in Chu inscriptional materials are of Kra–Dai origin. For example, the Chu graph for 'one, once' written as (? < OC *nnəŋ) in the E jun qijie 鄂君啟筯 bronze tally and in Warring States bamboo inscriptions, which represents a Kra–Dai areal word; compare Proto-Tai *hnïŋ = *hnɯŋ (Siamese ²²nɯŋ, Dai ³³nɯŋ, Longzhou nəəŋ^A etc.) 'one, once'.