"Acacia pycnantha" was first formally described by botanist George Bentham in the "London Journal of Botany" in 1842. The type specimen was collected by the explorer Thomas Mitchell in present-day northern Victoria between Pyramid Hill and the Loddon River. Bentham thought it was related to "A. leiophylla", which he described in the same paper. The specific epithet "pycnantha" is derived from the Greek words "pyknos" (dense) and "anthos" (flower), a reference to the dense cluster of flowers that make up the globular inflorescences. Queensland botanist Les Pedley reclassified the species as "Racosperma pycnanthum" in 2003, when he proposed placing almost all Australian members of the genus into the new genus "Racosperma". However, this name is treated as a synonym of its original name.