Brachypelma klaasi is sexually dimorphic, with females being larger and heavier than males. Female B. klaasi body size ranges from 50 to 75 mm, and weigh between 19.7 and 50 grams. Male weight ranges from 10 to 45 grams.

These spiders are colorful, by our standards. Brachypelma klaasi has a black carapace, tarsi, femora, and coxae, and orange-yellow metatarsi, tibias, and patellas. The hairs on the opistosoma are also orange-yellow in color. Although this sounds flashy, these spiders are actually well camoflauged in their natural environment, and are quite difficult to see on natural substrates.

Range mass: 10 to 50 g.

Other Physical Features: ectothermic ; bilateral symmetry

Sexual Dimorphism: female larger