Kadua is a genus of flowering plants in the family Rubiaceae. It comprises 29 species, all restricted to Polynesia. Twenty-two of these are endemic to the Hawaiian Islands. Some of the species are common at high elevation. Others are single-island endemics or very rare, and a few are probably extinct. Kadua affinis is widely distributed in Hawaii and is polymorphic. The type species for the genus is Kadua acuminata.
Kadua was formerly included in a broadly defined and polyphyletic Hedyotis, which encompassed, in addition to Kadua, species now placed in Oldenlandia, Oldenlandiopsis, Houstonia, and other genera. Hedyotis is now circumscribed more narrowly.