Dahl's law (German: Das Dahlsche Gesetz^[1]) is a sound rule in some of the Northeast Bantu languages that illustrates a case of voicing dissimilation. In the history of these languages, a voiceless stop, such as /p t k/, became voiced (/b d ɡ/) when immediately followed by a syllable with another voiceless stop. For example, Nyamwezi has -datu "three" where Swahili, a Bantu language that did not undergo Dahl's law, has -tatu, and Shambala has mgate "bread" where Swahili has mkate. Dahl's law is the reason for the name Gikuyu when the language prefix normally found in that language is ki- .

The law was named in 1903 by Carl Meinhof in his paper "Das Dahlsche Gesetz": in the paper, Meinhof explains that he named the rule after his pupil, the Moravian missionary Edmund Dahl, who reported it in 1897 when visiting the Wanyamwezi tribe in Urambo.^[1][2] It is productive in Sukuma, in the Nyanyembe dialect of Nyamwezi, most E50 languages (such as Kikuyu, Embu and Meru) and some J languages (such as Rwanda, Gusii and Kuria). In other languages the law is no longer productive, but there are indications that it once was (such as in Taita, Kamba/Daisũ, Taveta and Luhya/Logooli). In some neighboring languages (and in other dialects of Nyamwezi) words reflecting Dahl's law are found, but they appear to be transfers from languages in which the law is productive.^[3]

Dahl's law is often portrayed as the African equivalent of Grassmann's law in Indo-European languages.^[2] However, an analogue of Grassmann's law (which is aspiration, not voicing, dissimilation) has taken place in the Bantu language Makhuwa, where it is called Katupha's law.