The outlet plenum of a pebble bed modular-type gas-cooled nuclear reactor was optimized using three-dimensional Reynolds-averaged Navier-Stokes analysis and optimization techniques. A shear stress transport turbulence model was used as a turbulence closure. Two design variables for the optimization were selected: dimensionless displacement on the horizontal line and the angle of rotation about the center of gravity of the roof support block. The objective function was defined as a pressure drop between the inlet and the outlet of the outlet plenum. Latin hypercube sampling was used for selecting experimental design points within the design space. The objective function value was obtained at each design point through numerical analysis. The results show that the optimal design significantly improved the performance of the outlet plenum with respect to pressure drop. Through optimization, the pressure drop decreased by 11.8% compared to the pressure drop under the reference geometry.