This paper presents the effect of idealised unsteady tidal velocities on the performance of a newly-designed Horizontal-Axis Tidal Turbine (HATT) through the use of numerical simulations using Computational Fluid Dynamics (CFD). Simulations are conducted using ANSYS FLUENT implementing the Reynolds-Averaged Navier Stokes (RANS) equations to model the fluid flow problem. A steady flow case is modelling in a 2 m/s stream flow and the resulting performance curve was used as the basis of comparison for the unsteady flow simulations. A decrease in performance was seen for the unsteady flow simulation around peak TSR (TSR = 6) which has a cyclic-averaged coefficient of performance (CP) of 37.50% compared to the steady CP of 39.46%. Similar decreases in performance with unsteady flow was observed away from the peak performance TSR at TSR = 4 and TSR = 8. Furthermore, with unsteady flow that it was found that as the TSR increases, the difference between the cyclic-averaged CP and the steady flow CP drops. The effect of variations in the frequency and amplitude of the unsteady flow showed that a decrease in the cyclic-averaged CP was observed and this performance reduced with increasing frequency and increasing amplitude of unsteady incoming flows. For the cases studied here, unsteady flows are detrimental to the performance of the tidal turbine.