Plants shrubby or treelike, 1.3-4 m tall; trunks (when present) terete. Stem segments flattened, 10-30 cm long, 7.5-12.5 cm wide, obovate-oblong, oblong. or oblanceolate, narrowed basally, glossy green when young, thin, margins undulate towards the top; areoles 3-5 mm in diameter; spines 10-75 mm long, sparse, 1 or 2(or 3) per areole on stem segments but up to 12 per areole on trunk, erect or spreading, grayish, dark brown tipped, acicular; glochids 2-3 mm long, brownish; leaves conic, 2-4 mm, deciduous. Flowers 5-7.5 cm in diameter; outer tepals 8-25 mm long, 8-15 mm wide, obovate or broadly ovate,with red midribs and yellow margins, tops rounded or emarginate; inner tepals 23-40 mm long, 12-30 mm wide, obovate to oblong-obovate, yellow to orange, margins subentire, tops rounded, truncate, or muricate; filaments about 12 mm long, greenish; anthers about 1 mm long, pale yellow; styles 12-20 mm long, greenish; stigmas 6-10, 4.5-6 mm long,cream. Fruits 50-75 mm long, 40-50 mm in diameter, obovoid, reddish purple, umbilicus slightly depressed; seeds about 4 mm long, 3 mm wide, light tan, irregularly elliptic.
Opuntia monacantha is native to South America but has become widely established in tropical and subtropical regions. Its presence in Somaliland has not yet been documented. Thulin, in the Flora of Somalia, reported its presence in regions C1 and S2 of that work.