Panicum niihauense, a member of the grass family (Poaceae), is a perennial bunchgrass with unbranched culms (aerial stems) 50 to 125 cm (20 to 49 in) long. The leaf blades are flat, 15 to 35 cm (6 to 14 in) long and 0.7 to 1.9 cm (0.3 to 0.7 in) wide. The panicles (loosely branched inflorescences) are 13 to 35 cm (5 to 14 in) long. The panicle branches lie close to the main stem of the inflorescence (not spreading outward), and the spikelets are borne densely along the inflorescence branches. The spikelets, which contain two flowers, are 2.6 to 3.2 mm (0.1 to 0.13 in) long. This short-lived perennial species is distinguished from others in the genus by the shape of the inflorescence branches, which are erect and appressed, and the arrangement of the spikelets, which are densely clustered.