Marsh bedstraw is an erect forb, 20-60cm tall, with slender stems that are minutely and sparsely rough on the angles and not bearded. Its inflorescence is made up of a repeatedly branching cyme with 10-25 flowers above the leaflike bract. Pedicels are short and slender, mostly ascending at anthesis, widely spreading or somewhat reflexed in fruit. Corolla are white, 4mm broad, with 4 acute petals. Fruits are 2mm and smooth. Leaves are linear to narrowly oblanceolate, 5-15mm long, in whorls of in of 5-6, and blunt.