Stems 3-38 × (2-)4-10(-18) cm; ribs 8-13 (plant often reaching sexual maturity while still entirely tuberculate); areoles spaced 6-20 mm apart along ribs. Spines (8-)12-30 per areole; radial spines (8-)12-20(-27) per areole, straw colored or white, contrasting with all other spines, 9-35(-45) × 0.3-1 mm, adaxial 1-3+ spines of areole strongly flattened, bladelike or ribbonlike, to 4 times wider than thick, 16-75(-100) mm (usually absent from sexually immature plants); central spines (9-)13-42(-58) × (0.2-)0.4-1.5 mm. Flowers: inner tepals rose-pink to magenta, marked with scarlet or orange proximally, sometimes with white band or margins near bases; stigma lobes reddish to red-orange or pale reddish brown or yellowish. Seeds finely papillate with strongly convex testa cells. 2n = 22. Thelocactus bicolor varies geographically, and an indefinite number of segregate taxa may deserve recognition in Mexico.