Alder buckthorn is a non-spiny deciduous shrub, growing to 3–6 m (10–20 ft), occasionally to 7 m (23 ft) tall.
It is usually multistemmed, but rarely forms a small tree with a trunk diameter of up to 20 cm (8 in).
The bark is dark blackish-brown, with bright lemon-yellow inner bark exposed if cut.
The shoots are dark brown, the winter buds without bud scales, protected only by the densely hairy outer leaves.

The leaves are arranged alternately on 8–15-millimetre (⁵⁄₁₆–¹⁹⁄₃₂-inch) petioles.
They are ovate, 3–7 cm (1 ¹⁄₄–2 ³⁄₄ in) long by 2.5–4 cm (1–1 ⁵⁄₈ in) wide (rarely to 11 cm or 4 ¹⁄₄ in by 6 cm or 2 ¹⁄₄ in).
They have 6–10 pairs of prominently grooved and slightly downy veins and an entire margin.

The flowers are small, 3–5 mm (¹⁄₈–³⁄₁₆ inch) in diameter, star-shaped with five greenish-white acute triangular petals, hermaphroditic, and insect-pollinated, flowering in May to June in clusters of two to ten in the leaf axils.

The fruit is a small black berry 6–10 mm (¹⁄₄–¹³⁄₃₂ inch) in diameter, ripening from green through red in late summer to dark purple or black in early autumn, containing two or three pale brown 5-millimetre (³⁄₁₆-inch) seeds.
The seeds are primarily dispersed by frugivorous birds, which readily eat the fruit.
Alder buckthorn is a non-spiny deciduous shrub, growing to 3–6 m (10–20 ft), occasionally to 7 m (23 ft) tall. It is usually multistemmed, but rarely forms a small tree with a trunk diameter of up to 20 cm (8 in). The bark is dark blackish-brown, with bright lemon-yellow inner bark exposed if cut. The shoots are dark brown, the winter buds without bud scales, protected only by the densely hairy outer leaves.
The leaves are arranged alternately on 8–15-millimetre (⁵⁄₁₆–¹⁹⁄₃₂-inch) petioles. They are ovate, 3–7 cm (1 ¹⁄₄–2 ³⁄₄ in) long by 2.5–4 cm (1–1 ⁵⁄₈ in) wide (rarely to 11 cm or 4 ¹⁄₄ in by 6 cm or 2 ¹⁄₄ in). They have 6–10 pairs of prominently grooved and slightly downy veins and an entire margin.
The flowers are small, 3–5 mm (¹⁄₈–³⁄₁₆ inch) in diameter, star-shaped with five greenish-white acute triangular petals, hermaphroditic, and insect-pollinated, flowering in May to June in clusters of two to ten in the leaf axils.
The fruit is a small black berry 6–10 mm (¹⁄₄–¹³⁄₃₂ inch) in diameter, ripening from green through red in late summer to dark purple or black in early autumn, containing two or three pale brown 5-millimetre (³⁄₁₆-inch) seeds. The seeds are primarily dispersed by frugivorous birds, which readily eat the fruit.
Frangula alnus