Résumé : The efficiency in a horizontal axis tidal turbine (HATT),퐶푃,is the determinant factor for tidal energy system. Accordingly, predicting the 퐶푃of tidal turbines in the real sea environment iscriticalimportant 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 full-scale HATT. Anin-houseCFD approach,involving a modified wall-function approach, forapproximating the surface roughness due to barnacle fouling has been applied in order to predict the effectsof foulingon 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 퐶푃at 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 퐶푇due to the surface fouling canalsobefound. The results suggestthat the surface condition should be considered when scheduling routine maintenance to maintain the efficiency of tidal turbines.