Aloe ferox is a tall, single-stemmed aloe, that can grow to 10 feet (3.0 m) in height. Its leaves are thick and fleshy, arranged in rosettes, and have reddish-brown spines on the margins with smaller spines on the upper and lower surfaces. The leaf surfaces of young plants are covered in spines; however, as they get taller and less vulnerable to grazing, the leaves begin to lose most of their spines except for those along the leaf margins. Plants in the western part of its natural range tend to keep more of their leaf surface spines.
Its flowers are a uniform orange or red, and stand between 2 and 4 feet (0.61 and 1.22 m) above the leaves, in multi-branched inflorescences.
It is a variable species, and plants may differ physically from area to area, due to local conditions. This aloe is frequently confused with the related Aloe excelsa species, to the north, and they do look very similar when fully grown. However the flowers are different, with the racemes of Aloe excelsa being far shorter and slightly curved.
Altogether, the bitter aloe can be distinguished from its closest relatives: by its more compact, erect leaves with 6mm reddish-brown teeth on the margins and also on the keel of the leaf near the leaf tip; by their erect candelabra inflorescences, which bear up to eight very dense, cylindrical, symmetrical, 50–80 cm racemes; and by their un-curved, tubular flowers with brown inner segment tips.