The first diagnostic sirenian material from Madagascar and, more broadly, the first diagnostic pre-Pleistocene Cenozoic mammal material recovered from the island is reported. Eotheroides lambondrano is a new species of sirenian collected from middle Eocene nearshore marine deposits in the Mahajanga Basin of northwestern Madagascar. The recovered material consists of a nearly complete adult skull (including the first complete rostrum known for Eotheroides) and several portions of pachyosteosclerotic ribs. Diagnostic features of E. lambondrano include: primitive upper dental formula of 3.1.5.3, relatively large occlusal area of M2 and M3, long and narrow nasals, weak rostral deflection compared to other Eocene Dugongidae, well developed supraorbital processes, short infraorbital canal, anteroposteriorly short zygomatic-orbital bridge of maxilla, and a palate that is narrow anteriorly, creating a strongly bellshaped maxillary dental arcade. The cranium of E. lambondrano is similar to that of E. aegyptiacum from the middle Eocene of Egypt in aspects of both morphology and size but the upper molars of E. lambondrano are considerably longer and wider and it has longer, narrower nasals. The age and relatively primitive morphology of E. lambondrano suggests that it may represent the ancestral form from which more northerly species were derived.