Four new species and one new combination of Adenia are presented, along with a vegetative key and diagnostic characters of the Madagascan species. Additional notes are provided about unusual specimens and field observations. Adenia kigogoensis from Tanzania is shown to be distinct from A. stenodactyla (its putative closest relative) and A. digitata based on anther connation and other floral traits. The remaining new taxa are from Madagascar. Adenia litoralis has been observed from one coastal locality in northern Madagascar. It is distinctive in fruit size and leaf form. Adenia metamorpha is the only Madagascan taxon with a narrow cylindrical trunk and large napiform tuber; it is also known from only one locality in Madagascar in the Ankarana Reserve. Adenia mcdadiana is a robust liana with highly reduced glands and leaves that appear to be neotenic compared to its closest putative relative, A. sphaerocarpa. Finally, the position of A. stylosa has been clarified. This species was once treated as A. firingalavensis var. stylosa, and prior to that as A. epigea var. stylosa, but molecular and morphological data suggest it is separate from these species.