Cyphalonotus is a poorly studied araneid spider genus with species from Africa and Asia whose phylogenetic proximity remains unknown due to the paucity of morphological and molecular data. We here report on a taxonomic and evolutionary research on these spiders with three main objectives: i) to test the taxonomic composition of Cyphalonotus; ii) to test its phylogenetic placement; and iii) to place the male and female size variation of Cyphalonotus and related genera in an evolutionary context. Our original collection and field observations from Taiwan and China facilitated description of a new and a known species, and newly provided sequence data enable species delimitation, and phylogenetic analyses. The phylogenetic results reject all four classification hypotheses from the literature, and instead recover a well-supported clade Cyphalonotus + Poltys. We review the male and female size variation in Cyphalonotus, Poltys, and related genera. These data reveal that all known species of Poltys are extremely sexually size dimorphic (eSSD = females over twice the male size) reaching values exceeding 10-fold differences, while Cyphalonotus and other genera in their phylogenetic proximity are relatively sexually monomorphic (SSD < 2.0). This confirms an independent origin of eSSD in Poltys, one of multiple convergent evolutionary outcomes in orbweb spiders.