Rhizomes present; cauline stems terete, without dark spots, to 250 cm long. Turions absent. Nodal glands absent. Leaves submersed, alternate, more or less spiraling. Submersed leaves sessile; ligules inconspicuous, free from blade, light green to light brown, not fibrous, not shredding apically, deliquescent, convolute, 3.5–6.5 cm long; leaf blade lax, dark green, filiform to broadlyoblong, not arcuate, 0.9–7.6(–9.7) cm long, 7–40 mm wide, with 3–25 veins, without lacunae each side of midrib, the base truncate, clasping, without basal lobes, the margins entire to undulate, the apex mucronate or rarely aristate, without a bristle tip, not cucullate. Inflorescences unbranched, distal only; distal peduncles cylindrical, turgid, axillary or terminal, erect or rarely recurved (rarely), 1–7.3 cm long; spikes emersed, cylindrical, 4–48 mm long, the nodes 3–9, internodes 1.5–3 mm long. Flowers whorled; perianth spathulate, clawed, the claw 0.3–0.7 mm long, the blade 1.3–2.1 mm long, 0.7–1.9 mm wide; anthers 1–1.2 mm long; carpels 4. Fruiting gynophore absent. Fruits obovate, olive green to greenish brown, turgid, abaxially rounded, laterally rounded, 1.6–3 mm long, 1.3–2.2 mm wide; beak erect, 0.4–0.6 mm long; tubercules absent; embryo ca. one full spiral. 2n= 52.

Flowering summer--autumn. Waters, often brackish, of lakes, streams, rivers, and bays; 0--100 m; Greenland; St. Pierre; N.B., Nfld., N.S., Ont., P.E.I., Que.; Ala., Conn., Del., D.C., Fla., La., Maine, Md., Mass., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., Va., Vt.; Central America in Guatemala, Eurasia, Australia, Africa.

Four hybrids, Potamogeton perfoliatus ´ P. praelongus = P. ´ cognatus P. Ascherson & P. Graebner, P. perfoliatus ´ P. pusillus = P. ´ mysticus T. Morong, P. gramineus ´ P. perfoliatus = P. ´ nitens G. H. Weber (P. ´ subnitens J. O. Hagström), and P. alpinus ´ P. perfoliatus = P. ´ prussicus J. O. Hagström, have been described.