A new species of moss-mimicking mantis, Arria stanleyi sp. nov. is described based on female and male specimens collected in Tam Dao, Vietnam. The new species shows morphological characteristics vastly different from most other species in the genus Arria (Stal, 1877), with Arria muscoamicta (Unnahachote et al. 2021) having the most similar characteristics. Its discovery may outline a distant evolutionary branch compared to other species within the genus due to its geographic isolation and unique morphology. Genetic barcode is presented, and behavior and habitat are described.