Trachydoras is a genus of Doradidae (thorny catfishes) with five nominal valid species distributed in lowland areas of the Amazon, Orinoco, Paraná-Paraguay and Essequibo river basins of South America. A sixth species is described here as Trachydoras gepharti and diagnosed by five characteristics unique among congeners: mental barbels thick, tapered and profusely ornamented with many elongate fleshy papillae loosely arranged in 2–3 rows (vs. mental barbels thinner, papillae lacking or fewer, arranged in 1–2 rows); distinct columns of small, soft papillae along medial and lateral margins of gill filaments on all gill arches (vs. gill filaments lacking conspicuous papillae in congeners); gas bladder acorn-shaped (vs. cordiform) with smooth anterolateral shoulder (vs. shoulder with accessory diverticulum) and terminal diverticula medially united into singular, finger-like projection formed mostly by elongation of only one of the two posterior chambers (vs. both chambers elongated, contributing more or less equally to terminal diverticula in congeners or terminal diverticula absent or reduced in some specimens of T. nattereri and T. paraguayensis). Trachydoras gepharti is known from the Amazon and Orinoco basins where it often occurs syntopically with T. microstomus, T. nattereri and a separate undescribed species of Trachydoras. Like other species of Trachydoras, T. gepharti is specialized for vacuuming chironomid larvae from sandy substrates in medium to large river channels. Redescriptions and diagnoses are provided for the five nominal valid species of Trachydoras along with a key to identification and comments on characteristics used to diagnose the genus and delimit species. Designations include a lectotype (NMW 46375, 91.7 mm SL) for Trachydoras brevis (Kner 1853) and neotype (ANSP 178443, 100 mm SL) for T. nattereri (Steindachner 1881). The true holotype of T. microstomus (Eigenmann 1912) is identified as FMNH 118302 [ex. FMNH 53207, ex. CM 1650] and the specimen previously cataloged as the holotype (FMNH 53206) is identified as T. brevis.