Alnus acuminata grows up to 25m tall with a straight trunk up to 150cm thick. The bark has many yellowish lenticels. The leaves are simple, oval with toothed margins. The inflorescences are catkins, separate male and female flowers on the same tree. The male flowers are up to 12cm long and pendulous, while the smaller female flowers are green, erect and resemble a small cone. After wind fertilisation, the female flowers develop into 2cm long dehiscent, woody brown fruits. There are 80 to 100 winged seeds per fruit, and these are liberated when ripe, leaving the dried out fruit husks on the tree.