Swamp milkweed is similar to common milkweed except that it is smaller and ranges from deep pink to white. These plants are named for the thick milky sap which is exuded when the stem is broken. The individual flowers within the milkweed umbel are unusual, with the 5 petals bent backwards (downwards) and supporting 5 horns pointing in the opposite direction.

The smooth seedpod is up to 4 inches long, and is narrow and pointed. The pod splits along one side to release the seeds, which have long, silky fibers attached to aid in wind dispersal. Red-winged blackbirds use these fibers to attach their nests to vegetation, and ruby-throated humming birds use them to line their nests. Some birds eat the seeds.

Although milkweeds have been used extensively in folk medicine, they are noxious and can be toxic. The caterpillars of Monarch butterflies feed on this plant, which gives both the caterpillars and the butterflies an unpleasant taste and protects them from being eaten by birds.