The leaves of Phialanthus grandifolius are 4-8 x 1-2.5 cm, and indeed somet of the largest in this genus. Phialanthus hispaniolae is similar, and these may differ mainly geographically; and both of those species are similar to Phialanthus macrostemon.

This seems to be the most commonly collected species of Phialanthus, probably because of the collecting effort made on the serpentine formations of Puerto Rico.