The formosan alder is a trees up to 20 meters in height with dark gray-brown bark. The petiole is 1.2-2.2 cm, slender; leaves elliptic or oblong-lanceolate, rarely ovate-oblong, 6-12 × 2–5 cm, hairy in the axils of lateral veins beneath, almost hairless above, rounded or broadly cuneate base, irregularly minutely serrated margin, acuminate or acute apex; lateral veins 6 or 7 on each side of the midrib. It has one female inflorescence, or 2-4 in a cluster, ellipsoid, 1-2.5 cm; peduncle 3–5 mm.^[2]

It is found in Taiwan. It is a common species growing on riverbanks from near sea level to 2,900 metres (9,500 ft).^[2] It is commonly found in disturbed habitats as a pioneer species.^[3]

Alnus formosana flowers between May–June, fruiting between July–September.^[2]

This tree is used for soil improvement.^[3] It also finds use in gardens and as a windbreak. The tree trunks is also used in paper pulping and cultivating snow fungus and shiitake.^[4]

The Atayal people uses A. formosana as a cover crop after clearing a new field, the traditional wisdom being that the soil becomes rich when it is cut cleared again in 10 to 15 years.^[3][5] It is also used in the Pas-ta'ai ritual of the Saisiyat people.^[6]

Early Han settlers of Taiwan name some places after the occurrence of the plant, the belief being that its occurrence is linked to ground collapse.^[7]