Stems erect, subcircular in cross-section, 2–4 cm in diam., green. Ribs tuberculate, 4–6, not separated by conspicuous line at sulcus. Central spines 1–4, 10–20 mm long, 1.5–2 times as long as radial spines, red or white, ageing grey with black tips. Radial spines to 10 mm long. Flower buds with reddish hairs. Flowers 19–22 cm long. Pericarpel and lower hypanthium areoles not similar to stem areoles. Pericarpel green. Hypanthium green, scales red to reddish green, deltoid-lanceolate. Mid-hypanthium scales 2.6–4.2 mm long, 7.9–11.5 mm wide. Outer tepals pale reddish green. Inner tepals white. Fruits globose, slightly tuberculate, spineless, red, scales deciduous or sometimes persistent.
Of the three species of Harrisia naturalised in Australia, H. regelii is the least common. It is an environmental weed in Acacia harpophylla and Casuarina pauper woodland, as well as in Acacia harpophylla and Eucalyptus populnea woodland.
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