Alder buckthorn is a small tree or a spreading shrub, reaching 2 - 7 meters tall. The bark is dark gray with white speckles. The bark of the young trees is shining reddish brown. The bark of the older trees is gray, greyish-brown or pale. The leaves are alternately arranged, ovate, with an entire margin and with parallel veins.
The alder buckthorn flowers throughout the summer. Flowers and fruit can occur on the same plant simultaneously. The flowers are bisexual, small, shining-green, and arranged in groups of 2 - 7 at the leaf bases.