Navajo or Navaho (/ˈnævəhoʊ, ˈnɑːvə-/ NAV-ə-hoh, NAH-və-; Navajo: Diné bizaad [tìnépìz̥ɑ̀ːt] or Naabeehó bizaad [nɑ̀ːpèːhópìz̥ɑ̀ːt]) is a Southern Athabaskan language of the Na-Dené family, through which it is related to languages spoken across the western areas of North America. Navajo is spoken primarily in the Southwestern United States, especially in the Navajo Nation. It is one of the most widely spoken Native American languages and is the most widely spoken north of the Mexico–United States border, with almost 170,000 Americans speaking Navajo at home as of 2011.

The language has struggled to keep a healthy speaker base, although this problem has been alleviated to some extent by extensive education programs in the Navajo Nation. In World War II, speakers of the Navajo language joined the military and developed a code for sending secret messages. These code talkers' messages are widely credited with saving many lives and winning some of the most decisive battles in the war.

Navajo has a fairly large phonemic inventory, including several consonants that are not found in English. Its four basic vowel qualities are distinguished for nasality, length, and tone. Navajo has both agglutinative and fusional elements: it uses affixes to modify verbs, and nouns are typically created from multiple morphemes, but in both cases these morphemes are fused irregularly and beyond easy recognition. Basic word order is subject–object–verb, though it is highly flexible to pragmatic factors. Verbs are conjugated for aspect and mood, and given affixes for the person and number of both subjects and objects, as well as a host of other variables.

The language's orthography, which was developed in the late 1930s, is based on the Latin script. Most Navajo vocabulary is Athabaskan in origin, as the language has been conservative with loanwords due to its highly complex noun morphology.

The word Navajo is an exonym: it comes from the Tewa word Navahu, which combines the roots nava ('field') and hu ('valley') to mean 'large field'. It was borrowed into Spanish to refer to an area of present-day northwestern New Mexico, and later into English for the Navajo tribe and their language. The alternative spelling Navaho is considered antiquated; even the anthropologist Berard Haile spelled it with a "j" despite his personal objections. The Navajo refer to themselves as the Diné ('People'), with their language known (its endonym) as Diné bizaad ('People's language') or Naabeehó bizaad.

Navajo is an Athabaskan language; Navajo and Apache languages make up the southernmost branch of the family. Most of the other Athabaskan languages are located in Alaska, northwestern Canada, and along the North American Pacific coast.

Most languages in the Athabaskan family have tones. However, this feature evolved independently in all subgroups; Proto-Athabaskan had no tones. In each case, tone evolved from glottalic consonants at the ends of morphemes; however, the progression of these consonants into tones has not been consistent, with some related morphemes being pronounced with high tones in some Athabaskan languages and low tones in others. It has been posited that Navajo and Chipewyan, which have no common ancestor more recent than Proto-Athabaskan and possess many pairs of corresponding but opposite tones, evolved from different dialects of Proto-Athabaskan that pronounced these glottalic consonants differently. Proto-Athabaskan diverged fully into separate languages c. 500 BC.

Navajo is most closely related to Western Apache, with which it shares a similar tonal scheme and more than 92 percent of its vocabulary, and to Chiricahua-Mescalero Apache. It is estimated that the Apachean linguistic groups separated and became established as distinct societies, of which the Navajo were one, somewhere between 1300 and 1525. Navajo is generally considered mutually intelligible with all other Apachean languages.