Assamese or Asamiya (অসমীয়া [ɔˈxɔmija] ^ⓘ) is an Indo-Aryan language spoken mainly in the north-eastern Indian state of Assam, where it is an official language. It has long served as a lingua franca in parts of Northeast India. It has over 15 million native speakers and 8.3 million second language speakers according to Ethnologue.

Nefamese, an Assamese-based pidgin in Arunachal Pradesh, was used as a lingua franca before being replaced by Hindi; and Nagamese, an Assamese-based Creole language, continues to be widely used in Nagaland. The Kamtapuri language of Rangpur Division of Bangladesh and the Cooch Behar and Jalpaiguri districts of India is linguistically closer to Assamese, though the speakers identify with the Bengali culture and the literary language. In the past, it was the court language of the Ahom kingdom from the 17th century.

Along with other Eastern Indo-Aryan languages, Assamese evolved at least before the 7th century CE from the middle Indo-Aryan Magadhi Prakrit. Its sister languages include Angika, Bengali, Bishnupriya Manipuri, Chakma, Chittagonian, Hajong, Rajbangsi, Maithili, Rohingya and Sylheti. It is written in the Assamese alphabet, an abugida system, from left to right, with many typographic ligatures.

Assamese was designated as a classical Indian language by the Government of India on 3 October 2024 on account of its antiquity and literary traditions.

Assamese originated in Old Indo-Aryan dialects, though the exact nature of its origin and growth is not clear yet. It is generally believed that Assamese and the Kamatapuri lects derive from the Kamarupi dialect of Eastern Magadhi Prakrit though some authors contest a close connection of Assamese with Magadhi Prakrit. The Indo-Aryan, which appeared in the 4th–5th century in Assam, was probably spoken in the new settlements of Kamarupa—in urban centers and along the Brahmaputra River—surrounded by Tibeto-Burman and Austroasiatic communities. Kakati's (1941) assertion that Assamese has an Austroasiatic substrate is generally assumed—which suggests that when the Indo-Aryan centers formed in the 4th–5th centuries CE, there were substantial Austroasiatic speakers that later accepted the Indo-Aryan vernacular. Based on the 7th-century Chinese traveller Xuanzang's observations, Chatterji (1926) suggests that the Indo-Aryan vernacular differentiated itself in Kamarupa before it did in Bengal, and that these differences could be attributed to non-Indo-Aryan speakers adopting the language. The newly differentiated vernacular, from which Assamese eventually emerged, is evident in the Prakritisms present in the Sanskrit of the Kamarupa inscriptions.

The earliest forms of Assamese in literature are found in the 9th-century Buddhist verses called Charyapada the language of which bear affinities with Assamese (as well as Bengali, Maithili and Odia) and which belongs to a period when the Prakrit was at the cusp of differentiating into regional languages. The spirit and expressiveness of the Charyadas are today found in the folk songs called Deh-Bicarar Git.

In the 12th-14th century works of Ramai Pandit (Sunya Puran), Boru Chandidas (Krishna Kirtan), Sukur Mamud (Gopichandrar Gan), Durlabha Mallik (Gobindachandrar Git) and Bhavani Das (Mainamatir Gan) Assamese grammatical peculiarities coexist with features from Bengali language. Though the Gauda-Kamarupa stage is generally accepted and partially supported by recent linguistic research, it has not been fully reconstructed.

A distinctly Assamese literary form appeared first in the 13th-century in the courts of the Kamata kingdom when Hema Sarasvati composed the poem Prahlāda Carita. In the 14th-century, Madhava Kandali translated the Ramayana into Assamese (Saptakanda Ramayana) in the court of Mahamanikya, a Kachari king from central Assam. Though the Assamese idiom in these works is fully individualised, some archaic forms and conjunctive particles too are found. This period corresponds to the common stage of proto-Kamta and early Assamese.