Polyscias flynnii is a tree species that can reach heights of approximately 7 to 9 meters. It is well-branched above the middle, featuring secretory canals throughout most of its parts, and displays an andromonoecious (male and hermaphrodite flowers are on the same plant) reproductive strategy. The leaves of this species are compound and pinnate, arranged alternately along the stems. The leaf blades span 28–57 cm in length, with petiolules of lateral leaflets ranging from 2 to 22 mm and those of the terminal leaflet stretching from 17 to 50 mm. Each leaf comprises 5 to 9 narrowly ovate to elliptic leaflets, each measuring 4–8 cm in width and 7–17 cm in length. The lowermost pair of leaflets may sometimes be reduced in size. The leaflets’ apex ranges from acute to nearly rounded, with bases that can be acute, rounded, or subcordate and often oblique on lateral leaflets. The surfaces are densely covered in rusty brown to tawny farinose stellate pubescence, with the upper surfaces of the leaflets being glossy green and glabrous, featuring a contrasting whitish scurf. The mid-vein is yellow-green and farinose stellate pubescent below, with evident yellow-brown venation. Leaflet margins are entire and minutely revolute. The petioles are stout, measuring 7–17 cm long and about 4–8(–10) mm in diameter at the base. It produces flowers in axillary, paniculate-umbellate, and pendent inflorescences that are densely covered in brown farinose stellate pubescence. The primary axis of the inflorescence is stout, measuring 5–13 cm in length, with secondary axes bearing approximately 10 to 15 flowers. Flowers can be bisexual (perfect) or staminate, featuring a cupuliform calyx that is shallow at anthesis and expands in fruit. The corolla petals are lanceolate-triangular, yellow-green, fleshy, and densely farinose stellate pubescent on the dorsal surface. Stamens number 10 (or 11) in hermaphrodite flowers, with filaments approximately 4 mm long and anthers 3–3.5 mm long. In staminate flowers, filaments are shorter, and anthers measure about 2.5 mm in length. The ovary is inferior and 5-carpellate, surrounded by a concave nectar disk. The fruit is an urceolate (urn-shaped) drupe, measuring 15–20 mm in height and 10–16 mm in width, with a distinctly expanded calyx rim. Seeds are tan-brown, ovate to elliptic-compressed, and broader at the base, measuring 11 x 7 mm. It is found in steep, north-facing mesic to wet Ōhi‘a forests with a primarily open canopy between 585-1200 meters.