In a previous work [O. Ciftja, Phys. Lett. A 374 (2010) 981] we reported the exact calculation of the Coulomb self-energy of a uniformly charged two-dimensional square. The purpose of this brief extension of the earlier work is to report the corresponding exact result for the Coulomb self-energy of a uniformly charged three-dimensional cube. The calculations were carried out by using a straightforward generalization of the same mathematical method adopted in the earlier study. © 2010 Elsevier B.V. All rights reserved.