This species is characterized by medium-sized leaves, rather well developed bilobed stipules that have dense sericeous pubescence adaxially (on the stem side), pedunculate branched inflorescences with the flowers subsessile in a few groups, lobed short calyx limbs, white medium-sized corollas with a markedly swollen and bent base, and relatively large fruits. Plants of this species are atypicaly large for Palicourea, generally 8-14 m tall. The inflorescence axes are usually verticillate, or apparently so, and the bracts small. The pyrenes are broadly angled on the abaxial side. The fruits have been documented to be dispersed by Geoffroy's Woolly Monkey (Lagothrix cana) in Bolivia.

As circumscribed here Palicourea allenii is rather widely distributed and morphologically variable, and its distribution is somewhat unusual. The variation in leaf size, inflorescence size and branching, corolla size, specimen drying color, and some other features were analyzed by Taylor (2018). Palicourea allenii is quite commonly collected in Panama. This species is found mostly at relatively lower elevations, except in a few places it is found up to higher elevations and in western Colombia and northwestern Ecuador, it is only documented from these higher elevations (a few other Rubiaceae show similar range patterns).

Palicourea allenii is similar to Palicourea imbaburana, Palicourea vallis, Palicourea palenquensis, and Palicourea zakii, and has sometimes been confused with Palicourea horquetensis of southern Central America when the flowers are only in bud.