Tidal current turbines have undergone tremendous development on their scales and capacities in recent decades, but their further commercial application has encountered great challenges, including their high energy cost and short lifetime. The optimization design for cost reduction requires accurate and reliable turbine load information, and the load reduction control for life extension requires recognizable real-time fatigue load feedback. To avoid the difficulty and complication of direct load measurements, an indirect load measurement method for a large floating horizontal-axis tidal current turbine was proposed in this study based on the numerical relationship between the turbine loads and platform responses. The operation principle of the proposed method was defined in detail, and a turbine load measurement system was designed. To validate the feasibility of this measurement system, a measurement experiment for a 300-kW tidal current turbine on a floating platform was conducted. The turbine loads, consisting of the turbine thrust and fatigue loads, were estimated based on the measured information. In a comparison with the load information predicted by software simulation, good agreement, especially on the turbine thrust, was obtained. The estimated fatigue load signal indicated a high signal-to-noise ratio, which can provide reliable support for load control.