Lythrum salicaria is a herbaceous perennial plant, that can grow 1–2 m tall, forming clonal colonies 1.5 m or more in width with numerous erect stems growing from a single woody root mass. The stems are reddish-purple or red to purple and square in cross-section. The leaves are lanceolate, 3–10 cm long and 5–15 mm broad, downy and sessile, and arranged opposite or in whorls of three.
The flowers are reddish purple, 10–20 mm diameter, with six petals (occasionally five) and 12 stamens ( one stamen has up to 3,000 seeds), and are clustered tightly in the axils of bracts or leaves; there are three different flower types, with the stamens and style of different lengths, short, medium or long; each flower type can only be pollinated by one of the other types, not the same type, thus ensuring cross-pollination between different plants. The flowers are visited by many types of insects, and can be characterized by a generalized pollination syndrome.
The fruit is a small 3–4 mm capsule containing numerous minute seeds. Flowering lasts throughout the summer. When the seeds are mature, the leaves often turn bright red through dehydration in early
autumn; the red colour may last for almost two weeks. The dead stalks from previous growing seasons are brown.