In this paper, a hybrid meta-heuristic algorithm based on Harris Hawk Optimization (HHO) and Particle Swarm Optimization (PSO) is developed and used for the design of castellated beams. The total cost, including material and construction costs, is taken as the objective function, and the code requirements and geometry limitations have been considered in constraint functions. The proposed method is used to obtain the optimal solution of three examples. The solution is then compared with the results of five known methods. It is observed that the proposed algorithm achieves better results with a higher convergence speed as compared to the considered methods.