The Kadu languages, also known as Kadugli–Krongo or Tumtum, are a small language family of the Kordofanian geographic grouping, once included in Niger–Congo. However, since Thilo Schadeberg (1981), Kadu is widely seen as Nilo-Saharan. Evidence for a Niger-Congo affiliation is rejected, and a Nilo-Saharan relationship is controversial. A conservative classification would treat the Kadu languages as an independent family.

Blench (2006) notes that Kadu languages share similarities with multiple African language phyla, including Niger-Congo and Nilo-Saharan, suggesting a complex history of linguistic convergence and contact. However, more recently, Blench states that Kadu is almost certainly Nilo-Saharan, with its closest relationship being with Eastern Sudanic.

Like the Nilotic, Surmic, and Kuliak languages, Kadu languages have verb-initial word order. However, most other languages of the Nuba Mountains, Darfur, and the Sudan-Ethiopia border region have verb-final word order.

There are three branches:

Hall & Hall (2004), based on Schadeberg (1987), classify the languages as follows.

Dafalla (2000) compares 179 cognates in Kadugli, Kamda, Kanga, Katcha, Keiga, Kufa, Miri, Shororo-Kursi, and Tulishi. Dafalla's (2000) results are similarly to those of Schadeberg (1989).

Some Kadu quasi-reconstructions by Blench (2006):

Sample basic vocabulary for Kadu languages: