While biofouling is known to degrade the performance of marine energy conversion systems, prior experimental work has not explored this topic for cross-flow turbines. Here, we present experiments that investigate the impact of biofouling on turbine power output and structural loads. Using additive manufacturing, a three-dimensional scan of a barnacle was patterned onto the surface of turbine blades at three sizes and number densities, representing the progression from initial colonization to maturity. The impact of barnacles on turbine power output was found to be substantial and, for the most severe cases of fouling, the turbine does not produce power at any rotation rate. Conversely, barnacle fouling was found to have minimal impact on structural loading. To maintain generation capacity over extended periods, these results highlight the importance of antifouling coatings and proactive blade cleaning.