Alder buckthorn is a small tree or shrub that grows to a height of 2 to 7 metres. The outer bark is dark grey. The twigs are smooth and purplish brown with fine white streaks. The ovate leaves are alternately arranged on the stem and are slightly downy on the veins.
The alder buckthorn flowers throughout the summer. Flowers and fruit can appear on the same plant simultaneously. The small, star-shaped flowers contain male and female reproductive parts. The triangular petals are greenish white and are arranged in clusters of two to seven at the leaf base.