The title compound, [Co(C(2)H(8)N(2))(3)](CO(3))I·4H(2)O, crystallizes with a [Co(en)(3)](3+) cation (en is ethane-1,2-diamine), CO(3) (2-) and I(-) anions and four water mol-ecules in the asymmetric unit. In the cation, the three rings formed by the ethyl-enediamine units and the Co(III) metal ion are in slightly distorted twist conformations. Numerous O-H⋯O, N-H⋯O, N-H⋯I and O-H⋯I inter-molecular hydrogen bonds between the cation and two anions in concert with the four water mol-ecules dominate the crystal packing and create a supra-molecular infinite three-dimensional framework.