The Ndu languages are the best known family of the Sepik languages of East Sepik Province in northern Papua New Guinea. Ndu is the word for 'man' in the languages that make up this group. The languages were first identified as a related family by Kirschbaum in 1922.

Along with the Arapesh languages, Ndu languages are among the best documented languages in the Sepik basin, with comprehensive grammars available for many languages.

A diagnostic innovative feature in the Ndu languages is the replacement of the proto-Sepik pronoun *wun ‘I’ with proto-Ndu *an ~ *na.

Ambulas is the most populous language, with about 45,000 speakers, though Iatmül is better known to the outside world. There are eight to twelve Ndu languages; Usher (2020) counts nine:

Also sometimes distinguished are Keak (close to Iatmul or Sos Kundi), Kwasengen (or Hanga Hundi, close to Ambulas), Burui (close to Gai Kundi), and Sengo.

Most Sepik and neighboring languages have systems of three vowels, /ɨ ə a/, that are distinct only in height. Phonetic [i e u o] are a result of palatal and labial assimilation of /ɨ ə/ to adjacent consonants. The Ndu languages may take this reduction a step further: In these languages, /ɨ/ is used as an epenthetic vowel to break up consonant clusters in compound words. Within words, /ɨ/ only occurs between similar consonants, and seems to be explicable as epenthesis there as well, so that the only underlying vowels that need to be assumed are /ə/ and /a/. That is, the Ndu languages may be a rare case of a two-vowel system, the others being the Arrernte and Northwest Caucasian languages. However, contrasting analyses of these same languages may posit a dozen vowel monophthongs.

For Ndu languages, the glottalized low vowel /a̭/ is often written as /aʔa/. This does not signify /a/ followed by a glottal stop and another /a/.

Ndu languages mark first and second person possessors with -n, and third person possessors with -k.