Common sweet flag is a wetland perennial plant with a thick, spongy rhizome that is yellowish green on the outside. The sword-shaped leaves are 70 to 180 centimetres long and have a vertical midrib. The plant has a fleshy, ribbed stalk that bears a cylindrical spadix. The leaf-like spathe covering the stalk continues past the spadix to the same length as the leaves. The flowers are yellowish green and comprise six film-covered petals. Sweet flag flowers from May to June, and sometimes as late as July. The fruit is a dry, red berry. The plant propagates by rhizome division.
The sweet flag is typically found at the edges of small lakes and ponds and in marshes.