Plants erect, normally with few-many, closely parallel branches forming a narrow crown. Stems light to deep green, or blue-green especially when young; ribs 9-13; areoles copiously hairy; hairs deciduous to persistent, long or short. Spines 9-31 per areole; longest spines 10 mm. Flowers campanulate, 5-6 cm; outer tepals light green with brownish midstripes , ovate to obovate, apex acute to obtuse; inner tepals white; outermost ovate to elliptic, innermost oblanceolate to broadly linear. Fruits sometimes with 1-2 scales. Seeds 2 mm.

Flowering (Mar-)Aug-Sep; fruiting year-round. Upland tropical hardwood hammocks, 'cactus hammocks' (low elevation thorn scrub), sandy soils, nearly bare coral rock; of conservation concern; 0-10 m; Fla.; Mexico; West Indies.

In the flora, Pilosocereus robinii has been known historically from Key West to Key Largo, Florida, but is now restricted to Upper and Lower Matecumbe Keys, Long Key, and Big Pine Key. Pilosocereus keyensis and P. deeringii were originally described (as Cephalocereus species) from Key West and Lower Matecumbe Key, respectively, where they supposedly were endemic. Recent authors (D. F. Austin 1984; A. N. Lima and R. N. Adams 1996; R. P. Wunderlin and B. F. Hansen, www.plantatlas.usf.edu) have considered them to be within the range of variation of P. robinii. Although E. F. Anderson (2001) treated all of those named variants and others as merely insular forms of P. polygonus (Lamarck) Byles & G. D. Rowley, such treatment remains to be supported by a populational study throughout the West Indian range of the taxa.

In 1992, a clone identified as Pilosocereus bahamensis (Britton & Rose) Byles & G. D. Rowley was discovered in a rock hammock surrounded by mangrove swamp on Key Largo, Florida. E. F. Anderson (2001) also included that species as a synonym of P. polygonus.

When main stems fall to the ground, for whatever reason, the lateral branches develop into plants with the `unbranched´ pattern that was the basis for Small´s concept of Cephalocereus deeringii (D. F. Austin 1984).

According to L. D. Benson (1982), when the flowers initially open in the afternoon they may have an odor of onion or garlic.

Pilosocereus robinii is in the Center for Plant Conservation´s National Collection of Endangered Plants.