The Dagur, Daghur, Dahur, or Daur language, is a Mongolic language, as well as a distinct branch of the Mongolic language family,^[2] and is primarily spoken by members of the Daur ethnic group.

There is no written standard in use, although a Pinyin-based orthography has been devised; instead the Dagur make use of Mongolian or Chinese, as most speakers know these languages as well.^[3] During the time of the Qing dynasty, Dagur was written with the Manchu alphabet.^[4]

Dagur is a Mongolic language consisting of five dialects:^[5]

• Amur Dagur in the vicinity of Heihe (original homeland). About 400 people.
• Nonni Dagur on the west side of the Nonni River from south of Qiqihar up to Morin Dawa Daur Autonomous Banner. Speakers of Nonni Dagur are usually grouped into the following 4 dialects:
  • Morin Daba Dagur, in Morin Dawa Daur Autonomous Banner (Moli Daba) of Hulun Buir League, Inner Mongolia
  • Butha (Buteha) (Northern)^[6] Dagur, immediately south of Morin Dawa Daur Autonomous Banner
  • Tsitsikar (Southern)^[6] Dagur, in Tsitsikar (Qiqihar) City and surrounding areas
  • Mergen Dagur or Nenjiang Dagur, in Nenjiang County (formerly Mergen County) of Heilongjiang Province
• Hailar Dagur to the south-east of Hailar in Ewenki Autonomous Banner
• Sinkiang Dagur in Xinjiang in the vicinity of Tacheng

Dagur phonology is peculiar in that some of its dialects have developed a set of labialized consonants (e.g. /sʷar/ 'flea' vs. /sar/ 'moon'),^[7] while it shares palatalized consonants^[8] with most Mongolian dialects that have not been developed in the other Mongolic languages. It also has /f/, which is, however, limited to loan words.^[9] Word-final short vowels were lost^[10] and historically short vowels in non-initial syllables have lost phoneme status.^[11] Dagur is the only Mongolic language to share this development with Mongolian (i.e. Mongolian proper, Oirat, Buryat). Due to the merger of /ɔ/ and /ʊ/ with /o/ and /u/, vowel harmony was lost.^[12] According to Tsumagari (2003), vowel harmony is still a productive synchronic phonotactic aspect of Dagur in which initial syllable long vowels are divided into "masculine" (back), "feminine" (front), and neutral groups. Likewise, suffixal long vowels must agree in harmonic group with the root.

Dagur has a pronominal system that distinguishes between first person plural inclusive /bed/ and exclusive /baː/ and, even more archaic, it distinguishes between third person singular /iːn/ and plural /aːn/.^[13] While the phoneme /t͡ʃ/ (< *t͡ʃʰ) has been retained, the second person singular pronoun has become /ʃiː/ nevertheless,^[14] resembling a more thorough sound change in Khorchin Mongolian. The second person plural is retained as /taː/.^[14] The genitive and accusative have fused in some variants, becoming –ji, and the ablative may assume the form of the instrumental case. The old comitative has been lost, while the innovated comitative is the same as in Mongolian.^[15] In addition, several other cases have been innovated that are not shared by Mongolian, including a new allative, -maji.^[16]

Dagur has a fairly simple tense-aspect system consisting of the nonpast markers -/bəi/ and (marginally) -/n/ and the past forms -/sən/ and (marginally) /la/ and the non-finite imperfective marker -/d͡ʒa/-. These may be inflected for person. The attributive particle forms are limited to –/ɡʷ/ (< Written Mongolian -γ-a) for imperfective aspect and future tense, -sən (< -γsan) for perfective aspect, -/ɡat͡ʃ/ (< -gči) for habituality (instead of -daγ which used to fulfil this function) and -/mar/ for potential and probable actions. It has acquired a highly complex converbal system containing several innovations. Notably, -mar which is a participle in Mongolian serves as a converb as well.^[17]

It is estimated that out of Dagur's entire language vocabulary, over half is Mongolic in origin.^[21] Additionally, while Dagur has over 50% common Mongolic vocabulary, it has borrowed 5^[22] to 10% of its words from Chinese, as well as 10% of its words from Manchu, and a small number vocabulary borrowed from Evenki^[23] and Russian – leaving about 20% vocabulary that is specific to Dagur only.^[24]