Grevillea byrnesii is a shrub that typically grows to a height of. Its leaves are broadly egg-shaped, sometimes with the narrower end towards the base, long and wide, sometimes with up to seven coarse teeth on the edges. The flowers are arranged in loose cone-shaped or cylindrical groups in leaf axils or at the ends of branchlets, the rachis long, each flower on a downturned pedicel long. The flowers are orange, the pistil is long and the style has a yellow tip. Flowering occurs from May to August and the fruit is a glabrousfollicle long.