Leaves: stipules convolute; blade 0.9--5.4 cm ´ 0.2--2.5 mm; apex acute to obtuse, lacunae in 1--5 rows each side of midrib; veins 1--3(--5). Inflorescences: peduncles more than 3 per plant, cylindric to slightly clavate; spikes capitate to cylindric, continuous. Fruits ovoid, sides rounded, rarely concave; beak median, rarely toward adaxial edge. 2n = 26. Flowering and fruiting summer--fall. Shallow waters of lakes and streams; 0--2100 m; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask., Yukon; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mont., Nev., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., Vt., Va., Wash., Wis., Wyo.; Eurasia. Although Delaware and West Virginia lies within the mapped area, we know of no collections from that state. Potamogeton pusillus subsp. tenuissimus is the most common linear-leaved subspecies of the family in temperate North America. Whenever one finds a linear-leaved pondweed with 1--5 rows of lacunae on each side of the midvein, chances are that it is subsp. tenuissimus.

Only Potamogeton obtusifolius could be confused with the taxon, and it can be separated by having its cylindric inflorescence, whereas subsp. tenuissimus has a capitate inflorescence.