This article presents for the first time an OpenACC (Open accelerators)-aided Graphics Processing Unit (GPU)-based approach adopting a 3D moving window finite difference time domain (MW-FDTD) method for calculating lightning electromagnetic fields over large-scale terrains. The efficacy and accuracy of the proposed method are evaluated by comparing its results with the results obtained from a conventional FDTD method. The results show that the GPU-based MW-FDTD method is capable of providing highly accurate results 42 times faster than the conventional CPU-based FDTD method. Moreover, the proposed method required only 40% of the memory needed for carrying out the calculation compared to a conventional FDTD method, where M and Nx are the size of the entire domain in the conventional FDTD method and size of the divided blocks in the MW-FDTD method domain, respectively. The results show the applicability of the MW-FDTD-GPU method for dealing with lightning electromagnetic problems as well as large-scale electromagnetic compatibility (EMC) applications where the efficiency and minimization of the computational time and memory requirements are of great importance.