A perennial, submersed, dioecious aquatic herb with branched stems to 5 m long rooted in the soil; stems curve downwards near the water surface. Leaves spirally alternate, recurved. Tiny white female flowers reach above the water surface on long tubes; male flowers form in axils, break off and rise to the surface, where they bump into and pollinate female flowers. Fruit a capsule with ca. 9 seeds.

Water milfoils are submerged aquatic species with feathery leaves, although the flower spikes are held above the water. The flowers are small and unremarkable.

Stem thickened below the inflorescence to almost double the width of the lower stem, usually curved to lie parallel with the water surface; scales at the inflorescence nodes 2-3, black, distinct in fresh material; plants never forming turions, dying back to root crowns over winter.