Harrisia brookii grows as a shrub with richly branched, light green shoots 3 to 4 centimeters in diameter and reaches heights of up to 5 meters.

Its floral tube and pericarpel are covered with long, pointed scales.

The yellowish, ellipsoid to spherical fruits reach a diameter of up to 8 centimeters.

[1] Harrisia brookii is common in the Bahamas at elevations of 0-20 meters.

The specific epithet brookii honors the archivist Herbert A. Brooke from the Bahamas.