This letter presents a novel method to estimate the grid impedance based on stationary discrete wavelet packet transform (SDWPT). The proposed method uses a steady-state technique, by injecting an interharmonic current into the grid and measuring the voltage response at the point of common coupling to estimate the grid impedance. The proposed method employed a standard three-phase photovoltaic system interconnected to the grid to validate its effectiveness experimentally. Comparisons with a discrete Fourier transform- and continuous wavelet transform-based impedance estimation approaches demonstrate the performance of proposed method. Besides, the proposed SDWPT-based impedance estimation provided accurate experimental results, which make it viable for real-time applications.