Abstract: To propel the application of a bottom-hinged flap breakwater in real sea conditions, a two-dimensional computational fluid dynamics numerical model was conducted to investigate the pitching motion response and wave attenuation in random waves. First, the flow velocity distribution characteristic of the pitching flap at typical times was summarized. Then, the effects of random wave and flap parameters on the flap’s significant pitching angle amplitude θ_s and hydrodynamic coefficients were investigated. The results reveal that θ_s and wave reflection coefficient K_r values increase with increasing significant wave height H_s, random wave steepness λ_s, and flap relative height. As H_s and λ_s increase, the wave transmission coefficient K_t increases while the wave dissipation coefficient K_d decreases. Additionally, K_t decreases with increasing flap relative height. With increasing equivalent damping coefficient ratio, θ_s and K_t decrease, while K_r and K_d increase. The relationships between λ_s and flap relative height on the one hand and θ_s, K_r, K_t, and K_d in random waves on the other hand are compared to those in regular waves. Based on the equal incident wave energy and the equal incident wave energy flux, the pitching flap performs better in the wave attenuation capability under random waves than in regular waves. Finally, the dimensionless parameters with respect to random wave and flap were used to derive the K_r and K_t formulae, which were validated with the related data.