Eucalyptus rhomboidea is a mallet that typically grows to a height of but does not form a lignotuber. It has smooth greyish to brownish bark that is shed in ribbons. Young plants have stems that are square in cross-section and egg-shaped to elliptical leaves that are sessile, glaucous and arranged in opposite pairs. Adult leaves are lance-shaped, the same shade of green on both sides, long and wide, tapering to a petiole long. The flower buds are arranged in leaf axils in groups of seven on an erect, unbranched peduncle long, the individual buds on pedicels long. Mature buds are oval to rhomboid, about long and wide with a conical to slightly beaked operculum. Flowering occurs from September to October and the flowers are pale yellow. The fruit is a woody, cup-shaped to funnel-shaped capsule, glaucous at first and with the valves protruding strongly.
