Waves interact with currents in tidal channels with the resulting wave–current environment largely determining the loads experienced by tidal stream turbines. Over a tidal cycle, the magnitude and direction of the current velocity changes and hence so does the combined wave–current conditions the turbines must operate within. Here we demonstrate this effect experimentally, generating a realistic irregular wave case in both following (in the same direction as the waves) and opposing currents prior to assessing the resulting loads on a fully instrumented 1:15 scale tidal turbine model aligned with the current direction. Large changes in the environmental conditions, along with the turbine performance and loads, are demonstrated through the presentation of temporal, spectral and statistical outputs. The experimental results demonstrate that the full-scale equivalent significant wave height changes from 2.25 m in zero current to 6.11 m in 3.2 m/s opposing current and $$1.56\hbox { m}$$ in $$3.2\hbox {m/s}$$ following current. The corresponding standard deviations of measured turbine parameters for the opposing condition range between 215 and 260% of the following case, and between 340 and 565% of the current-only measurements. Hence, when waves are present, significantly greater fatigue damage will be accumulated during one-half of the tidal cycle. The mean values, however, appear to be unaffected by the presence of waves suggesting that the overall turbine performance is unaltered. These results demonstrate the requirement to understand the combined wave–current environment and to test and de-risk tidal stream turbines for operation in both following and opposing wave–current conditions. Significant additional insight is gained into the nature of loads experienced by tidal turbines in irregular wave conditions, a scarcely documented phenomenon.