Allomantispa Liu et al., 2015 is the only extant genus of the mantidfly subfamily Drepanicinae from Eurasia, being endemic to the Oriental region. It currently consists of only two species from southeastern Tibet and northern Myanmar, respectively. Here we describe a new species of this rarely observed mantidfly genus, namely Allomantispa coniprocessa sp. nov. from Yunnan, China. Interestingly, the new species displays distinctive sexual dimorphic characters on the pronotum. The molecular identification reveals obvious genetic divergence between the allopatric individuals of the new species. Their morphological variations are illustrated and compared. In addition, some characters with taxonomic or phylogenetic significance present on the pronotum, the abdomen and the genitalia in Allomantispa are also discussed. Our finding updates the morphological and distributional information on Allomantispa and implies a more diverse fauna of this genus in the Oriental region than previously assumed.