The efficiency in a horizontal axis tidal turbine (HATT), ??, is the determinant factor for tidal energy system. Accordingly, predicting the ?? of tidal turbines in the real sea environment is critical important to achieve the maximum performance of HATTs. However this performance is under great threat caused by marine fouling. And the understanding of the fouling effect is still barely known. This paper focuses on the study of the roughness effect due to biofouling on the performance of a HATT. A computational fluid dynamics (CFD) based unsteady Reynolds Averaged Navier-Stokes (RANS) simulation model was developed to predict the effect of biofouling on a fullscale HATT. An in-house CFD approach, involving a modified wall-function approach, for approximating the surface roughness due to barnacle fouling has been applied in order to predict the effects of fouling on the HATT performances. CFD simulations were conducted in different fouling scenarios for a range of tip speed ratios (TSR). The effect of surface fouling proved to be drastic resulting in up to 13% decrease in power coefficient ?? at the design operating condition (TSR=4). The effect proved to be even more severe at higher TSRs, resulting in narrower optimal operating TSR regions. However, reduced thrust coefficients ?? due to the surface fouling can also be found. The results suggest that the surface condition should be considered when scheduling routine maintenance to maintain the efficiency of tidal turbines.