Phialiphora Groeninckx is described as a new herbaceous genus of Rubiaceae, endemic to NW Madagascar. The genus belongs to the predominantly herbaceous tribe Spermacoceae, and comprises two species: P. bevazahensis Groeninckx and P. capitulata Groeninckx. Phialiphora is unique within Spermacoceae in having heart-shaped placentas distally attached to the septum with a stalk. The generic name refers to the involucrate head-like inflorescences. Molecular data, based on atpB-rbcL, petD, rps16, and trnL-trnF sequences, demonstrate that Phialiphora belongs to the Hedyotis-Oldenlandia complex of the tribe Spermacoceae. The genus shares a common ancestor with the Madagascan endemics Astiella Jovet, Thamnoldenlandia Groeninckx and Amphistemon Groeninckx. Our study supports the hypothesis that the current diversity of Spermacoceae on Madagascar is the result of several independent colonisation events since the Eocene, most likely by long-distance dispersal from the African continent. A key to the genera of Spermacoceae on Madagascar is provided.