Quercus humboldtii is an evergreen tree which grows to a height of 25 meters and a diameter of 1 meter, with buttresses of up to 1 meter. Its bark is reddish grey or grey and fissured, breaking into squares and flaking. The leaves simple, alternate and lanceolate, up to 10–20 cm long, and clustered at the ends of the branches. The flowers are small, yellow, and unisexual, with a racemic inflorescence. Male flowers are numerous, with long-styled female flowers in a cupula. The fruit is a light brown, ovoid capsule, or acorn, with a leathery pericarp, 20–25 mm in diameter and 50–70 mm long, resting on a scaly cupule. Only one fruit per cupule is developed, and the inside of the acorn shell is woolly.