Great Chickweed is a native perennial that grows about 6-12 inches tall and is usually unbranched completely or sparingly so. It produces both fertile and infertile shoots, the latter of which develop later in the year and do not product flowers. It has a central stem that is light green to pale purplish green and is slightly pubescent. The leaves are opposite and are up to 3 inches long. Each leaf tapers gradually to a sessile base while its tip may be either acute or blunt. Lower leaves are more likely to have blunt tips than upper leaves.

Flowers are about 1/2 inches across and consist of 5 green sepals, 5 white petals (each deeply bifurcated, giving the appearance of 10 petals), a white ovary with 3 styles in the apex, and 10 stamens with reddish brown anthers. The flowers bloom from mid to late spring and last about a month. Each flower is eventually replaced by a ovoid seed capsule. Small colonies of plants are occasionally formed.