Herbaceous perennial. The flowers are many and tiny, light green, on the lower part of a long, narrow, taillike spadix (a spikelike structure holding many tiny flowers). A tightly wrapping, green, leaflike spathe (modified bract) shelters the flowers. Blooms April–June. There is only one leaf, which is divided into 5–15 lance-shaped sections. The fruits are clustered berries that turn from shiny green to brilliant scarlet. The rootstock us a flattened, round corm, often large.
Similar species: Jack-in-the-pulpit (A. triphyllum) has leaves with no more than 3 leaflets (instead of 5–15), and its spathe has white and brown lengthwise markings and a rather blunt tip. The fruits of the two species are indistinguishable.