Swamp agrimony is a tall native perennial that grows in sunny to lightly shaded wet habitats, including swamps, floodplain woods, soggy thickets, prairie swales, and roadside ditches. Its fibrous, rhizomatous roots allow it to form small colonies. The stout, upright stems are unbranched or sparingly branched, covered with long white to light brown hairs, sometimes sticky to the touch.

Leaves are long (up to 2 ft) and compound, with 9–17 narrow primary leaflets and 1–4 pairs of smaller leaflets nestled between. Leaflets are typically arranged opposite or sub-opposite along the central stalk, giving the leaf a balanced, feathery appearance. Primary leaflets measure 2–3 inches long, narrowly lance-shaped to elliptic, with wedge-shaped bases, pointed tips, and coarsely toothed margins. The upper surface is yellow-green and smooth, while the underside is short-hairy, especially along the veins. Secondary leaflets are much smaller, usually less than 1 inch long. Both the stalk and midrib of each compound leaf are hairy. At the base of each leaf are large, fan-shaped stipules that are coarsely toothed or lobed.

The flowering stem ends in a tall, spike-like raceme 3⁄4 –2 1⁄2 ft long, sometimes accompanied by smaller racemes from the upper leaf axils. Flowers are tiny (about 1⁄4 in. across) and widely spaced along the raceme. Each flower has five yellow petals, about 10 stamens, and a central pistil surrounded by a tubular green calyx with 10 ribs. Flowers open from the bottom upward, with only a few open at once, giving the plant a prolonged but modest blooming period in mid to late summer.

Fruits are small burs, about 1⁄8–1⁄4 in. across, with hooked bristles around the rim of the persistent calyx. These easily cling to clothing, fur, or feathers, aiding seed dispersal. Immature fruits are green, turning brown when mature. Each fruit contains 1–2 seeds.