The Sasak language (/ˈsɑːsɑːk/ SAH-sahk; Base Sasaq, IPA: [ˈbasə ˈsasak], Sasak script: ᬪᬵᬲᬵᬲᬓ᭄ᬱᬓ᭄; Indonesian: Bahasa Sasak [baˈha.sa ˈsasak]) or Sasaknese is spoken by the Sasak ethnic group, which make up the majority of the population of Lombok, an island in the West Nusa Tenggara province of Indonesia. It is closely related to the Balinese and Sumbawa languages spoken on adjacent islands, and is part of the Austronesian language family. Sasak has no official status; the national language, Indonesian, is the official and literary language in areas where Sasak is spoken.

Some of its dialects, which correspond to regions of Lombok, have a low mutual intelligibility. Sasak has a system of speech levels in which different words are used depending on the social level of the addressee relative to the speaker, similar to neighbouring Javanese and Balinese.

Not widely read or written today, Sasak is used in traditional texts written on dried lontar leaves and read on ceremonial occasions. Traditionally, Sasak's writing system is nearly identical to Balinese script.

Sasak is spoken by the Sasak people on the island of Lombok in West Nusa Tenggara, Indonesia, which is located between the island of Bali (on the west) and Sumbawa (on the east). Its speakers numbered about 2.7 million in 2010, roughly 85 percent of Lombok's population. Sasak is used in families and villages, but has no formal status. The national language, Indonesian, is the language of education, government, literacy and inter-ethnic communication. The Sasak are not the only ethnic group in Lombok; about 300,000 Balinese people live primarily in the western part of the island and near Mataram, the provincial capital of West Nusa Tenggara. In urban areas with more ethnic diversity there is some language shift towards Indonesian, mainly in the forms of code-switching and mixing rather than an abandoning of Sasak.

Austronesian linguist K. Alexander Adelaar classified Sasak as one of the Malayo-Sumbawan languages group (a group he first identified) of the western Malayo-Polynesian family in a 2005 paper. Sasak's closest sister language is Sumbawa and, with Balinese, they form the Balinese-Sasak-Sumbawa (BSS) subgroup. BSS, Malayic (which includes Malay, Indonesian and Minangkabau) and Chamic (which includes Acehnese) form one branch of the Malayo-Sumbawan group. The two other branches are Sundanese and Madurese. This classification puts Javanese, previously thought to belong to the same group, outside the Malayo-Sumbawan group in a different branch of the western Malayo-Polynesian family.

The Malayo-Sumbawan proposal, however, is rejected by Blust (2010) and Smith (2017), who included the BSS languages in the putative "Western Indonesian" subgroup, alongside Javanese, Madurese, Sundanese, Lampung, Greater Barito and Greater North Borneo languages.

Kawi, a literary language based on Old Javanese, has significantly influenced Sasak. It is used in Sasak puppet theatre, poetry and some lontar-based texts, sometimes mixed with Sasak. Kawi is also used for hyperpoliteness (a speech level above Sasak's "high" level), especially by the upper class known as the mènak.

Eight vowels appear in Sasak dialects, contrasting with each other differently by dialect. They are represented in Latin orthography by ⟨a⟩, ⟨e⟩, ⟨i⟩, ⟨o⟩ and ⟨u⟩, with diacritics sometimes used to distinguish conflated sounds. The usual Indonesian practice is to use ⟨e⟩ for the schwa, ⟨é⟩ for the close-mid front vowel, ⟨è⟩ for the open-mid front vowel, ⟨ó⟩ for the close-mid back vowel and ⟨ò⟩ for the open-mid back vowel.