Shrubs, often treelike, erect or old plants with branches usually ascending, clambering, or prostrate, sparingly branched or unbranched. Roots diffuse (sometimes tuberlike). Stems unsegmented, green, long cylindric, 100-600 × 2.5-6 cm, glabrous; ribs 8-12, rounded, low, less than 1 cm deep, shallowly to indistinctly tuberculate; areoles ca. 2 cm apart along ribs, circular to oval, short woolly, sometimes subtended by small, subulate, deciduous leaves; areolar glands not apparent; cortex and pith mucilaginous. Spines 6-17 per areole, usually porrect to ascending, white to pinkish or yellow to light brown, aging gray, sometimes with tips darker or yellowish, acicular, straight, ± terete, 10-40 × 0.5-0.75[-1.5] mm, hard, smooth and glabrous; radial spines and central spines not clearly distinguishable. Flowers nocturnal, remaining open next day, lateral or terminal on stems at least 1 year old, funnelform, 12-20[-25] × 8-12 cm; outer tepals green to reddish or purplish, linear to lanceolate or narrowly oblong, 50-60 × 4.5-6 mm, margins entire; inner tepals white to pinkish, 60 -75 × 12-20 mm, margins entire or denticulate; ovary usually conspicuously tuberculate, scaly, spineless or spines represented by soft and silky to stiff hairs [or spiny]; scales entire; stigma lobes 8-15, yellow-green to white, 6-9 mm. Fruits indehiscent or splitting irregularly from apex toward base, green to yellow, red, or orange-red, spheric to ovoid-spheric, [30-]40-75[-80] × [30-]40-75[-80] mm, usually spineless (or spines bristlelike) [or slender acicular]; scales deciduous [sometimes persistent]; pulp white; floral remnant usually persistent. Seeds black, broadly ovoid or broadly ellipsoid, 2-3 × 1.5 mm, warty. x = 11.

In lieu of a detailed study of Harrisia, this treatment respects earlier taxonomies based in large part on the fruit color and the colors of the hairs in the areoles of the flowers and fruits. Both characters may prove to be variable within taxa of Harrisia and therefore not be taxonomically useful.

Harrisia was traditionally classified with columnar cacti of North America and Central America, e.g., have kinship with Acanthocereus, Selenicereus, or Hylocereus, which also form long-tubed, nocturnal flowers. Recent DNA analyses have demonstrated that Harrisia belongs instead to a large South American clade of columnar cacti (R. S. Wallace and A. C. Gibson 2002), with which they share axillary tufts of silky hairs on the flower tube.

The basis for E. F. Anderson´s (2001) inclusion of Harrisia eriophora in the flora, in addition to the species recognized here, is not apparent. He recognized H. donae-antoniae, an invalid name for a local Florida variant of H. aboriginum, in the synonymy of H. gracilis (Miller) Britton. He apparently did not intend, however, to include H. gracilis as native to Florida because he listed its geographic distribution as 'Jamaica.'

The fruits of several species of Harrisia are sweet and edible.