In this paper, optimum design of three-dimensional (3D) multi-story reinforced concrete (RC) structures is obtained using an improved version of the plasma generation optimization (IPGO) algorithm. The reinforcing bars and geometry of the cross-sections are considered as discrete variables that are chosen from a prearranged section database. The objective function is obtained from the cost of the framework, steel and concrete. The moment and shear frames are selected to design according to the American Concrete Institute’s Building Code (ACI 318-8) requirements. The effectiveness of the proposed algorithm is investigated in the optimum design of 3D RC frames subjected to lateral seismic forces according to ASCE 7 requirements. Also, moment frames and their shear frame counterparts are considered and compared for optimal design. The results indicate a good performance of the proposed algorithm to find an optimal solution for 3D RC frames.