The coastal lowland rain forests of eastern Africa are well known for their high levels of plant endemism. A new genus of Annonaceae, Mwasumbia, is described from Tanzania, underscoring this high biodiversity and represented by a single species, Mwasumbia alba. The new genus presents several morphological characters suggesting a close relationship to two other African genera, Greenwayodendron and Polyceratocarpus. Maximum parsimony and Bayesian molecular phylogenetic analyses based on two plastid markers, rbcL and the trnL-trnF region, strongly support the close relationship of these three genera to one another as well as to two other African genera, Annickia and Piptostigma. Together these five genera form a moderately supported clade within the so-called short-branch clade of Annonaceae. A detailed morphological and palynological comparison between Mwasumbia and the four other genera shows that this new genus exhibits a combination of features unique within this group: intermediate tertiary leaf venation, exclusively bisexual flowers, slightly imbricate sepals, valvate petals, outer and inner petals equal in length, numerous stamens, four carpels, few and uniseriate ovules, few and sessile monocarps, and verrucate sulculate pollen grains. Phylogenetic analyses also suggest the genus Piptostigma, as currently circumscribed, to be paraphyletic. An IUCN conservation status of VU D2 is proposed, reflecting the narrow distribution of the single species.