Abstract: Objectives The motion response of floating tidal current turbine systems in waves and currents involves complex interactions including hydrodynamics, mooring dynamics, and platform-turbine coupling effects. A fully coupled, full-scale numerical model is developed using STAR-CCM+ to investigate the system behaviour under random wave conditions. Methods The overset grid technique, in combination with dynamic fluid-body interaction (DFBI) modelling, is utilised to simulate system motions. Free surface is captured through the volume of fluid (VOF) method, while mooring line dynamics are represented using the lumped-mass approach. An integrated numerical model is established to incorporate the semi-submersible platform, turbines, and mooring system. Results The results demonstrate that integration of a single tidal current turbine significantly reduce pitch motion while suppressing low-frequency surge response and natural-frequency response of pitch and heave motions. The platform motion is found to amplify the fluctuation amplitude of the tidal turbine's hydrodynamic coefficients. Spectral analysis reveal wave-frequency dominance in thrust and power coefficient peaks. In multi-turbine configurations, slightly higher thrust values are observed in downstream turbines compared to upstream positions. Conclusions The findings of the present study provide fundamental insights for the optimisation of floating tidal energy converter designs.