Fluviphylax palikur, new species, is described from the rio Taparabu, rio Oiapoque basin, northern Brazil. It differs from all other species of the genus by the possession of numerous anal fin rays and vertebrae and by the shape of fourth and fifth ceratobranchials and second pharyngobranchial. Five synapomorphies shared by all species of Fluviphylax except F. palikur, in addition to characters supporting Fluviphylax monophyly, support a hypothesis of a sister-group relationship between the new species and a clade consisting of the remaining species of the genus. Miniaturization and reductive morphological conditions in Fluviphylax are discussed in a phylogenetic context. Miniaturization in Fluviphylax is hypothesized to correspond to a single event, with reductive conditions originating at the base of the Fluviphylax clade and in different intrageneric lineages. A list of Neotropical miniature freshwater fish described in the last 10 years is presented.