Gymnocalycium horstii is a distinctive large cactus, minimally spined with a green ball-shaped body. Usually solitary or slowly clustering as they age. Two subspecies are recognized, the nominate form and subsp. buenekeri (Swales) P.J.Braun & Hofacker. The latter was previously considered a good species, genetic studies are required to see if it should be raised to species level again.
Stem: Smooth, globoid, shiny green, up to 20(-25) cm in diameter.
Ribs: 5-6 cm wide, rounded, somewhat acute, deeply furrowed.
Spines: Few (usually 5 of which one pointing downward others to the sides) , yellowish white, sparse, straight to slightly curved, definite and very irregular, up to 3 cm long.
Flowers: White, pale pink, pink or pale purplish up to 11 cm long and in diameter.
Fruits: Ovoid, blue-green.