Clustered bur-reed is an aquatic forb, 2-6dm tall, with stout, floating to erect, stems. It has a simple inflorescence, occasionally with a basal branch. Flowers are unisexual, it has several sessile pistillate heads, 1.5-2cm thick when ripe, staminate head is solitary (or sometimes 2) and abuts pistillate heads below. Achenes have a brown, shiny, fusiform body, 3-4 mm, slightly constricted below the middle, tapering to a straight or slightly curved beak of 1.5-2mm. Leaves are flat or weakly keeled, 3-8mm wide. Bracteal leaves are dilated at the base.
Distinguishing characteristics include its 1-2 staminate heads that abut the upper pistillate head and its shiny achene.