We show through numerical investigations that the ground-state correlation energies of atomic ions follow an unexpectedly simple scaling relation, E c ≈ Z 4/3 f c (Z/N), where N is the number of electrons, Z is the atomic number, and f c is a universal function, for which an analytic expression with a one-parameter fit can be provided. The relation agrees well with several sets of correlation energies obtained from different methods for atomic ions with N = 2, . . . ,18 and Z = 2, . . . ,28. Moreover, our relation gives a good agreement with neutral atoms up to N ≈ 90. Our main result is readily applicable to estimating correlation energies of heavy elements, for which there are no available data in the literature. The simplicity of the relation may also have implications in the development of correlation functionals within density-functional theory.