Eriophorum vaginatum is a species of flowering plant in the sedge family Cyperaceae. It occurs throughout the subarctic and temperate regions of the Northern Hemisphere. It prefers wet, acidic, peaty soil and is often dominant in bogs, poor fens, and the Arctic tundra regions of northern Russia and Canada. The specific epithet vaginatum, which means "having a sheath", refers to an important character used to recognize this species. Both the scientific name Eriophorum vaginatum and the common name sheathed cottonsedge emphasize this fact. In Britain and Ireland (and elsewhere), the species is referred to as hare's-tail cottongrass (or simply hare's-tail). The name tussock cottongrass gained in popularity after its publication in Flora of North America in 2002.

Eriophorum vaginatum is a member of the unispicate group of cottongrasses since it has one (not multiple) spikelets. It is similar in appearance to the unispicate species Eriophorum brachyantherum and Eriophorum callitrix, all three of which have densely tufted stems. Eriophorum vaginatum is distinguished by having spreading scales at the base of the inflorescence, a conspicuously inflated sheath on its flowering stem, and a tussock-forming growth habit.

Some botanists recognize two (or even three) subspecies. One of those subspecies, Eriophorum vaginatum subsp. spissum, is widespread throughout northeastern North America but the full extent of its range is unknown. It differs from the typical subspecies based on the shape of its spikelet, the color of its scales, and length of its anthers. In Canada, the subspecies is known as dense cottongrass, a characteristic that is likewise true of all subspecies.

Eriophorum vaginatum is a perennial, tussock-forming sedge. It has a cespitose (tufted) growth habit, with multiple flowering stems at the base of the plant. A stem may reach up to 60 cm (24 in) tall, with a persistent brown sheath at its base. Basal leaves are approximately 1 mm wide and noticeably shorter than the flowering stem. There are 1–3 leaves along the length of the stem, each reduced to a sheath (no leaf blade). The uppermost sheath is conspicuously inflated. The inflorescence is a single erect spikelet at the tip of the stem. The spikelet bears numerous florets, each covered by a scale (a type of bract in sedges and grasses). At the base of the inflorescence, there are 10 or more empty scales (with no flowers), the lowermost scales becoming spreading or reflexed at maturity. The flower parts emerge from the axils of the floral scales. After pollination, each floret develops 10 or more white bristles that elongate to 2 cm at maturity. The fruit is a small brown nutlet no more than 3.5 mm long.

For identification purposes, the cottongrasses divide into two groups based on the number of spikelets. Eriophorum vaginatum is a member of the unispicate group since it has one (not multiple) spikelets. It is similar in appearance to the unispicate species Eriophorum brachyantherum and Eriophorum callitrix. All three species have densely tufted stems (cespitose) and 10 or more empty scales at the base of the inflorescence. Eriophorum vaginatum is distinguished by having white-margined empty scales with the lowermost scales spreading or reflexed (not appressed to ascending), a conspicuously inflated sheath, and a tussock-forming growth habit.

Eriophorum vaginatum subsp. spissum is very similar to the typical subspecies described above, differing in a few characters only:

Subspecies spissum is a smaller plant with a more compact tussock. It occurs throughout northeastern North America but the full extent of its range is unknown.

Eriophorum vaginatum, the type species of genus Eriophorum, was first described by the Swedish botanist Carl Linnaeus in 1753. Linnaeus based his diagnosis on specimens with smooth sheathed culms collected in the cold, barren regions of Europe.