Sylheti (Sylheti Nagri: ꠍꠤꠟꠐꠤ, síloṭi, pronounced [sílɔʈi] ^ⓘ; Bengali: সিলেটি, sileṭi, pronounced [sileʈi]) is an Indo-Aryan language spoken by an estimated 11 million people, primarily in the Sylhet Division of Bangladesh, Barak Valley of Assam, and northern parts of Tripura in India. Besides, there are substantial numbers of Sylheti speakers in the Indian states of Meghalaya, Manipur, and Nagaland as well as diaspora communities in the United Kingdom, the United States, Canada and the Middle East.

It is variously perceived as either a dialect of Bengali or a language in its own right. While most linguists consider it an independent language, for many native speakers Sylheti forms the diglossic vernacular, with standard Bengali forming the codified lect. Some incorrectly consider it as a "corrupt" form of Bengali, and there is a reported language shift from Sylheti to Standard Bengali in Bangladesh, India and the diaspora; though Sylheti has more vitality than Standard Bengali among the diaspora in the United Kingdom.

Sylheti is eponymously named after Sylhet, referring to the dialect or language spoken of that area. According to Grierson (1903) the vernacular was called Sylhettia by the Europeans after the town of Sylhet. Though the speakers at that time referred to it as Jaintiapuri, Purba Srihattiya, or Ujania with the latter meaning "the language of the upper country".

Sylheti is also spelt or known as Sylhetti (or Sileti), Sylheti Bangla and Siloti (also spelt in Syloti or Syloty).

Sylheti belongs to the Eastern Indo-Aryan languages that evolved from Magadhi Prakrit. The lowlands around Sylhet were originally inhabited by ancient Khasi people (Austroasiatic); and the earliest known Indo-Aryan settlements were made in the 6th century under Kamarupa king. The Sylheti language was known as the language of the ancient Surma Valley and the Barak Valley in the sixth century CE, so it is assumed that this language is nearly 15 centuries old. Sylhet (Srihatta) then emerged as a center of lowland territorialism after the 10th century. The 11th century Bhatera grants from the Srihatta kings Kesavadeva and Isanadeva were written in Sanskrit. Another notable copper plate inscription was found in the village of Paschimbhag in Rajnagar, Moulvibazar that was issued by King Srichandra during the 10th century.

The Muslim Conquest of Sylhet in 1303 CE extended the migratory movements of Muslims from western lands, who settled among the native population and greatly influenced the local language. Thus Sylheti derived a large number of words from Persian and Arabic, indicating the Perso-Arabic influence on the vernacular. A script was developed in the region called Sylheti Nagri, which primarily focused on disseminating Sufi poetry, known as puthi. Its earliest known work had been written during the 1600s, called Bhedsar by Syed Shah Husayn Alam. The literature was transcribed in the standard form of late Middle Bengali, it was similar to the Dobhashi idiom though phonologically was strongly influenced by Sylheti. The script was read and taught culturally among households and was not institutionalised, as the Islamic dynasties who ruled over Bengal established Persian alongside Arabic as the official languages. Printed texts of the script reached its peak during the late 19th century, however its use became obsolete by around the middle of the 20th century.

In 1903, Grierson reported that Sylheti was spoken only around Sylhet town of the then Sylhet district (now Sylhet Division and Karimganj district in Assam), and that among the Indo-Aryan speakers in the district, about 33 per cent spoke this language.

The earliest appearance of a documentation of Sylheti vocabulary was in the Government Report on the History and Statistics of Sylhet District by T. Walton, B.C.S. in 1857, which contained a list of peculiar words used in Sylhet. Many terms that were listed here differ from modern Sylheti – highlighting its evolution. In 1868, another short glossary of local terms in various districts of the Dacca Division (which included Sylhet) were written up and compared to standard Bengali to allow ease in understanding local vernaculars. Despite being annexed to the Assam Province during colonial rule, Sylheti speakers felt a linguistic affinity with the rest of Bengal. Bengali literature had some influence from Sylheti, popular songwriters or poets such as Hason Raja or Shah Abdul Karim, significantly contributed to the literature. Sylhet was reunited with Bengal following a referendum in 1947.