is similar to the other barrel cactus of California, ferocactus cylindraceus
, though the two species occupy distinct, non-overlapping ranges, so can readily be distinguished. The former species is found across large areas of Baja California; in the US it is more restricted, to the coastline around San Diego, and nearby hills, northwards only as far as Oceanside and Carlsbad.
The San Diego barrel cactus takes the form of a neat globe, never elongating into a column. It has between 13 and 21 ribs (occasionally more), and reaches a maximum diameter of around 10 inches. Areoles bear between 10 and 24 spines; the four central spines are flattened, tapering, slightly curved, while the radial spines are shorter and much thinner. One central spine, angled slightly downwards, is thicker and longer than the others, up to 2 inches. All spines are deep red-pink at first, maturing to yellow and gray. The yellow to reddish flowers appear in late spring.