Strychnos decussata is a small to medium tree, 3-12 m high. It has a dense crown with many stems. The bark is smooth or wrinkled and dark purplish grey. Branchlets are in opposite pairs, knobbly, sometimes with raised white dots with no spines. The leaves are small, 15-50 x 10-30 mm, oppositely arranged, hard, shiny, dark to pale green. The leaf blade is curved upwards. The leaf margin is entire, rounded or blunt with a sharp tip. The flowers are small, creamy white, scented and formed in branched clusters on old wood in October to December. The fruits are asymmetric with sharp tips and thin skin, and are orange, round to oval, small, 17 mm in diameter.