A hybrid finite-difference time-domain (FDTD)/Haar multiresolution time-domain (MRTD) technique for the time-domain analysis of microwave structures is proposed in this paper. The salient features of the presented algorithm are, first, its inherent stability that stems from the matching of the dispersion properties of FDTD and Haar MRTD and, second, its applicability to arbitrarily high wavelet order MRTD schemes. Thus, the application of the MRTD technique to the modeling of open structures and inhomogeneous circuit geometries is facilitated. In particular, the straightforward implementation of perfectly matched layer type and Mur's absorbing boundary conditions is attained. The fact that the proposed interface involves no spatial or temporal interpolations or extrapolations indicates its potential to efficiently connect FDTD and Haar MRTD.