Castanopsis microphylla is a tree in the family Fagaceae. The specific epithet ' is from the Greek meaning 'small-leaved'.
Castanopsis microphylla grows as a tree up to tall with a trunk diameter of up to . The bark is smooth, occasionally flaky. The coriaceous leaves measure up to long. Its ovoid nuts measure up to long.
Castanopsis microphylla is endemic to Borneo. Its habitat is lowland dipterocarp to lower montane forests up to elevation.