This species is characterized by shortly petiolate, medium-sized, generally ovate leaves with well developed domatia; well developed, papery stipules 9-14 mm long that are lacerate on the top; subsessile, subcapitate, few-flowered inflorescences; a truncate calyx limb 1.5-2 mm long; and a corolla with the tube ca. 12 mm long and the lobes 5-6 mm long and smooth abaxially. The fruits have not been conclusively documented (but see discusison below). The stipules are apparently initially tubular, and then often split along one or two sides. The identity of this species is not entirely clear, but the type matches a few other specimens the same region.
Another, more commonly collected group of plants in eastern, Amazonian, lowland Ecuador and Peru is similar to the type of Rudgea cryptantha, and appears from their aspect to be distinct but cannot be fully separated by any characters (e.g., Coello 127, Zaruma 647, both MO; Huamantupa 16086, F). These have apparently simlar corollas and smaller but generally smaller stipules, 6-10 mm long, and a calyx limb that appears to vary from lobed to truncate and 0.5-2 mm long, and white subglobose fruits 5-6 x 5-6 mm. These are provisionally included here but their aspect, with smaller leaves on extenstively branched plants, but further study with field observations will be needed to delimit Rudgea cryptantha. A number of Rubiaceae species seem to show notable morphological diversification in the Andean foothills of eastern Ecuador and northeastern Peru, and this may be another example.