Nothocestrum longifolium is a shrub or small tree reaching heights up to 6 meters. The branches are slender and ascending, featuring a smooth, glabrous surface. The leaves are simple and alternate, typically spaced relatively widely apart, though some are clustered at the branch tips. These leaves have a lanceolate to elliptic-oblong shape, measuring 10–32 cm long and 3.5–8.5 cm wide. The apex of the leaves varies from obtuse to acute or acuminate, while the base is cuneate. The upper surfaces of the leaves are glabrous or become glabrate shortly after, and the lower surfaces range from glabrous to moderately pubescent with a whitish-to-reddish hue. The leaves are relatively thin and chartaceous with entire margins. The flowers appear in axillary clusters, typically one to three but occasionally up to six. The flowers are bisexual, actinomorphic (radially symmetric), and pedicellate, featuring a tubular calyx 8–12 mm long and unevenly 4-lobed. The corolla is greenish-yellow, with the tube being pale yellow, not longer than the calyx, and silky pubescent in bud. There are four stamens, partly exserted, with extremely short to absent filaments, and linear, acute anthers that open longitudinally. The superior ovary is 2-celled and ovoid, with numerous ovules, an erect style, and a 2-lobed clavate, including stigma. The fruit is an orange berry, fleshy, and elongate or fusiform in shape, either entirely or partly enclosed by the calyx. The seeds are irregular, with a pitted seed coat, a curved embryo, and the presence of endosperm. It predominantly inhabits wet forests, though it can also be found in mesic and diverse mesic forests between 36-1,620 meters.