Swamp milkweed is a perennial herb. Stems smooth, tall, flexible, branching. Sap milky latex. Flowers mostly terminal (positioned at the tops of the stems), in many loose umbels (each rounded cluster arises from the same point); pink or rarely white, with a delicate fragrance. Blooms June–September. Leaves mostly opposite, narrowly lance-shaped, to 6 inches long and about 1 inch wide. Fruits slender pods, to 4 inches long, in pairs, smooth but usually slightly hairy, bearing seeds with silky parachutes.
Similar species: Swamp milkweed can be distinguished from similar milkweeds by its pink-flowered umbels (flower clusters) positioned at the tops of the plant stalks (not arising from the sides of the stems), relatively narrow leaves, and moist bottomland habitat.