The near lack of vertebrate fossils from the Cenozoic of Madagascar has left many of the details regarding the origin and evolution of the island's extant faunas unknown. However, recent fossil discoveries from Madagascar's nearshore marine deposits have begun to elucidate this mystery. These finds include sharks, bony fish, turtles, crocodylians, a middle Eocene sirenian (Eotheroides lambondrano), and the island's first fossil dolphin. We report here at least three (possibly four) different early (or possibly later) Miocene dugongid sirenians recovered from the island of Nosy Makamby, Mahajanga Basin, northwestern Madagascar. These include (1) a fragmentary braincase originally attributed to the genus Halitherium but here reidentified as a previously named species known only from Libya (Rytiodus heali; Dugonginae); (2) a newly named genus and species (Norosiren zazavavindrano) interpreted as a primitive relative of Xenosiren (Dugonginae); (3) a probable dugongine not yet identified with any known species; and (4) a taxon reported here as Metaxytherium cf. krahuletzi (Halitheriinae), the first Neogene halitheriine credibly reported from the Indian Ocean basin. This pattern of shallow marine environments harboring multispecies sirenian paleofaunas is seen elsewhere in the world, and these three or four contemporaneous sirenians represent the first glimpse into Madagascar's sea cow diversity during the Miocene. This specific time period is a poorly known and critical interval for interpreting Madagascar's past, and these specimens are potentially highly significant for reconstructing sirenian evolutionary and biogeographic history. Surprisingly, this sirenian fauna, so far, shares no genera with the roughly contemporaneous and relatively nearby one from Kutch, western India.