Peperomia subpetiolata is a fleshy herb characterized by its stout and erect ascending stems that are greenish with reddish-purple splotches. These stems can be quite large, measuring about 60–150 cm in length and 8–20 mm in diameter, with internodes ranging from 3 to 4 cm and up to 12 cm in the lower part of the stem. They are either unbranched or have a few branches in the upper part and are either glabrous or sparsely hirsute with spreading hairs about 1 mm long. The leaves are simple and whorled, with 5–8 per node. They feature linear-lanceolate to narrowly elliptic blades that measure 12–20 cm in length and 1.5–4 cm in width. The apex of the leaves is long-acuminate, while the base is narrowly cuneate to attenuate. The leaf surfaces are glabrous or sparsely hirsute on the veins, with the upper surfaces being dark green and the lower surfaces pale green. The leaves are relatively thin yet coriaceous and dry to chartaceous. Their margins are entire, palmately 5-veined or pinnately veined with 2–4 pairs, with veins impressed on the upper surface. Petioles are short, 0.5–1 cm long, and mostly glabrate. It produces flowers in spikes that are 8–12 cm long, with the rachis being 3.5–4 mm in diameter. These flowers are densely congested, terminal, and axillary, with peduncles approximately 2–3.5 cm long and glabrous. The flowers are bisexual (perfect) and lack a perianth, with each flower subtended by a peltate bract. The calyx (sepals) and corolla (petals) are absent, and the plant has two stamens. The ovary is superior, turbinate, 1-carpellate, and 1-celled, with the apex being somewhat oblique; it contains one orthotropous, erect ovule and features 1–4 terminal stigmas. The fruit is subglobose, about 1 mm in diameter, and papillose-viscid with sessile glands. It contains a single seed per fruit with a scanty endosperm and a copious starchy perisperm. It grows in mesic forests between 1,250-1,370 meters.