Quantitative time-lapse seismic monitoring can provide crucial information for enhanced oil recovery, geologic carbon storage, and enhanced geothermal systems. Recently developed double-difference elastic-waveform inversion has the potential to quantitatively monitor reservoirs using seismic reflection data. Because the approximate location of a reservoir or a target monitoring region is usually known, we incorporated this knowledge as prior information into double-difference elastic-waveform inversion. Using numerical examples of synthetic time-lapse models, we found that our new method can quantitatively monitor the changes of elastic properties within reservoirs. Therefore, the double-difference elastic-waveform inversion with prior information on the location of a monitoring region is a promising tool for quantitatively monitoring reservoir properties’ changes.