Closterium setaceum is characterized by its elongate, slightly curved, needle-shaped cells. Typically long and slender, the cells have smooth, unadorned walls. Each cell contains two axial, deeply lobed chloroplasts with pyrenoids and a centrally located nucleus in the isthmus. The pointed, tapering cell ends enhance its needle-like appearance. During cell division, a new semi-cell forms between the existing ones. Closterium setaceum exhibits slow, gliding movements, likely facilitated by mucilage secretion.