Surface roughness in channel and pipe flows is typically associated with higher wall shear stress and increased drag. However, the deliberate introduction of appropriately designed surface deformations can alternatively produce a significant reduction in drag [1]. These drag reduction methods have long been known but have been applied in an ad hoc manner. For example, sailing riblets and drag-reduction surface treatments used on aircraft. However, there remains a knowledge gap in terms of what parameters create a true optimal design for such constructions, and what the true drag reduction potential is. This work determines the optimal geometry of streamwise riblets to reduce drag and associated pressure drop across a planar channel. This optimization is performed using Direct Numerical Simulation (DNS), resolving all the unsteady turbulent flow features. A streamwise sinusoidal wall boundary is applied on the upper and lower surfaces, and the design parameters include the amplitude, phase, and wavelength of the upper and lower surfaces. Optimization is performed using the Mesh Adaptive Direct Search (MADS) algorithm, and the optimal design yields a 30% increase in flow rate with the same pressure drop and characteristic cross-sectional area as a flat channel.