Peperomia sandwicensis is a fleshy herb known for its distinctive reddish-purple to dark-purple stems at the base. These stems ascend to weakly erect from a short repent base, ranging in length from 10 to 30 cm and about 5 to 7 mm in diameter at the base. They typically exhibit dichotomous or verticillate branching above the middle, with internodes spanning 2 to 7 cm, covered in strigillose hairs (0.2–0.5 mm long) or sometimes hirtellous, with older stems becoming glabrate. The leaves of this species are simple and arranged oppositely or whorled with 2 to 6 per node. The blade shapes vary from rhombic-ovate to obovate, elliptic-obovate, or suborbicular, measuring 1.5 to 4 cm long and 1 to 2 cm in width. The apex of the leaves is rounded to obtuse with a broadly cuneate base. The upper surfaces are mostly glabrous, except near the margins where they are strigillose; the lower surfaces are evenly strigillose. The leaf colors vary, with upper surfaces being dark green or sometimes dark purplish green and lower surfaces red to dark red or sometimes pale green. The leaves are thick, firm-fleshy, and dry to chartaceous to coriaceous. The edges are flat and ciliate. Leaves are typically palmately 3-veined, sometimes with five veins, and the outermost pair is inconspicuous. Petioles are 0.5 to 1 cm long, also strigillose or occasionally hirtellous. It produces flowers in spikes, usually solitary and terminal, sometimes with a few laterals. The spikes are 3 to 7 cm long with a rachis about 1.5 to 2 mm in diameter and are glabrous. The densely congested flowers lack a perianth, and a peltate bract subtends each. There are no calyx or corolla. The flowers have two stamens. The ovary is superior, ovoid, 1-carpellate, 1-celled with an oblique apex, containing one orthotropous (upright with the micropyle), erect ovule; the stigmas are subterminal and number from 1 to 4. The fruits are broadly ovoid, about 0.8 mm long, and papillose-viscid with sessile glands. Each fruit contains one seed with a scanty endosperm and a copious, starchy perisperm. It is found in mesic valleys and wet forests between 250-1,220 meters.