Integration of the finite-difference time-domain (FDTD) method into the iterative multiregion (IMR) technique, an iterative approach used to solve large-scale electromagnetic scattering and radiation problems, is presented in this paper. The idea of the IMR technique is to divide a large problem domain into smaller subregions, solve each subregion separately, and combine the solutions of subregions after introducing the effect of interaction to obtain solutions at multiple frequencies for the large domain. Solutions of the subregions using frequency domain solvers has been the preferred approach as such; solutions using time domain solvers require computationally expensive bookkeeping of time signals. In this paper, we present an algorithm that makes it feasible to use the FDTD method, a time domain numerical technique, in the IMR technique to obtain solutions at a prespecified number of frequencies in a single simulation. A hybrid method integrated into the IMR technique is also presented in this paper. This hybrid method combines the desirable features of the FDTD method and the method of moments (MoM) to solve radiation problems more efficiently. As a result, a considerable reduction in memory storage requirements and computation time is achieved.