Accumulation of biological growth or biofouling can significantly influence the measurement accuracy of a buoy, unless it is cleaned on frequent intervals. This paper addresses the hydrodynamic impact of biofouling on met-ocean buoys moored in coastal zones. The analyses are mostly based on a Brazilian Navy buoy on the coast of Rio de Janeiro – Brazil. After only four months moored in the warm waters of the Guanabara Bay, the same buoy gained 25% in weight due to fouling, with additional important modifications on the shape and drag below the waterline. Initially, historical data measured by the buoy was analyzed to observe changes in its response over time. A motion response factor (lateral motion/vertical motion) was calculated from the data that showed a significant impact of biofouling for particular wave conditions. Next, OpenFOAM is used to perform RANS simulations for the buoy, at three different wave scenarios, in both clean and fouled condition to observe the difference in hydrodynamic response. The results indicated that the response amplitude operator varies notably for low and high-frequency cases between cleaned and fouled buoy. Our paper suggests that standard guidelines related to buoy cleaning intervals for biofouling and uncertainty applied in their measured readings may not be equal for all conditions. Buoys placed near the coast with warmer waters are more prone to biofouling and might have higher uncertainty in their measured data.