Arria muscoamicta Unnahachote & Shcherbakov, sp. nov. is described based on a male from central Thailand. This is the first record of Arria Stål, 1877 from the country. The new species is closely allied to A. leigongshanensis (Ge & Shen, 2008) from China, differing by the absence of prozonal tubercles, the elongated pronotum, nine tibial anteroventral spines, and the truncated hindwings. The new species is a moss-camouflaging mantis living at high altitude. The taxonomic problems of the genus are briefly discussed.