Zanthoxylum armatum is a small aromatic tree or large shrub up to 6 m high with incurved reddish brown prickles; bark grey-brownish, scabrate. Leaves compound, imparipinnate, ca. 20 cm long; rachis glabrous or rust-colored pubescent; petiolules ca. 2-4 mm long; lamina ca. 3-8 x 1-3 cm, ovate-lanceolate or elliptic to oblong, oblique at base, acuminate at apex, glandular-crenate along margins, chartaceous, glabrous; secondary nerves 5-20 pairs, inarching near margins. Cymes terminal on short lateral branchlets, ca. 4-10 cm long, paniculate, pubescent; Male flowers ca. 0.5 mm long, pubescent; perianth uni- or irregularly biseriate, segments 6-8, ca. 1 mm long, yellowish, ovate-lanceolate, acute or acuminate, glabrous; stamens 6-8, exserted; filaments ca. 1.5-2.5 mm long; anthers ca. 1 mm long, yellowish or reddish-purple, ovoid-oblong; disk ca. 0.5 mm long, pulvinate; Female flowers: pedicels ca. 2 mm long; ovary 1-3 or sometimes 4-carpellate, ca. 1.5 mm long, each carpel ovoid-subglobose, glandular-punctate; style ca. 0.5 mm long; stigma capitate; fruiting pedicels ca. 1-3 mm long. Follicles 1-3 with 2, 1 or 0 caducous abortive carpels, ovoid-subglobose, apiculate with persistent stylar base, 3-4 mm, postular. Seeds ca. 3 mm across, black, ovoid, smooth.